Surface modulation of inorganic layer via soft plasma electrolysis for optimizing chemical stability and catalytic activity

Journal Pre-proofs Surface modulation of inorganic layer via soft plasma electrolysis for optimizing chemical stability and catalytic activity M.P. Kamil, W. Al Zoubi, D.K. Yoon, H.W. Yang, Y.G. Ko PII: DOI: Reference:

S1385-8947(19)33029-3 https://doi.org/10.1016/j.cej.2019.123614 CEJ 123614

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

26 June 2019 5 November 2019 24 November 2019

Please cite this article as: M.P. Kamil, W. Al Zoubi, D.K. Yoon, H.W. Yang, Y.G. Ko, Surface modulation of inorganic layer via soft plasma electrolysis for optimizing chemical stability and catalytic activity, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123614

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Surface modulation of inorganic layer via soft plasma electrolysis for optimizing chemical stability and catalytic activity M. P. Kamil1, W. Al Zoubi1, D. K. Yoon1, H. W. Yang2, and Y. G. Ko1* 1Materials

Electrochemistry Group, School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea 2Pohang Institute of Metal Industry Advancement Pohang 37666, Republic of Korea

*Corresponding author: E-mail address: [email protected] (Y.G. Ko)

The present study reports the formation of alumina-based inorganic layer via soft plasma discharges for optimizing chemical stability and catalytic activity together. Alkaline silicate electrolyte with complexing agent is formulated to ‘soften’ the destructive nature of plasma discharges in water. The presence of complexing agent having large denticity activates the swarms of soft plasma events, resulting in a less porous architecture than that with simple structure due to a homogenization of electrical field. Accordingly, such inorganic layer comprises stable aluminosilicate since chemical transformations are facilitated to a greater extent by soft plasma discharges. The conformal inorganic layer in the present work demonstrates protective and catalytic features together on account of a synergy between compact microstructure and dynamic surface topography caused by residual tracks of plasma discharges. The reformation of electrical double layer after disruptive events, which underlines the formation of soft plasma discharges, is discussed on the basis of quantum chemical calculation and impedance analysis.

Keywords: plasma electrolysis; Al2O3; corrosion; methylene blue; H2O2

1. Introduction Over the last few decades, the rapid progress in state-of-the-art technology has stimulated the advent of high-performance materials with sufficient durability to be employed in real-time conditions where structural and functional aspects should be satisfied [1–5]. One of the emerging strategies to achieve such goal is plasma electrolysis (PE) [6–8], which has laid the groundwork to design inorganic layers for structural protection against corrosion [9–12] and wear [13,14], as well as for functional applications such as photocatalysis [15,16], electrocatalysis [17], and biomedics [18]. PE reforms the surface of a material in a wet electrolytic cell by utilizing myriads of micro-scale plasma discharges initiated at the interface between the electrolyte and material surface [6,7,19–21]. By applying a powerful electrical polarization, reactive plasma discharges are generated in liquid electrolyte under atmospheric pressure, offering a greater potential for large-scale manufacturing than those established under vacuum. The key to unlock the potential for creative material designs by PE resides in the control of plasma discharges accompanying the electrochemical formation of inorganic layers. The plasma-driven growth in this scenario is a complicated intertwine of electrochemical, thermal, and plasma reactions taking place simultaneously [7,20–22]. It is regarded as a general fact that process kinetics in PE is affected heavily by the competition of constructive and destructive nature of plasma discharges [6,21,23,24]. Under high electrical field, the dielectric arcs trigger the formation of inorganic layers outwards by volcanic-like discharge channels and inwards by field-assisted oxygen diffusion [21,23]. On the other hand, an excessive temperature (~3500 K) and pressure (2.5 bar) around plasma discharges [25,26] results inevitably in micro-sized pores and channels that are detrimental to the structural integrity of inorganic layers. Although a large number of research works have been dedicated to the remediation of these

defects by physical sealing of micro cavities [27–29], the destructive effect of plasma discharges itself remains practically untreated. To this end, ‘soft’ sparking regime, which is defined loosely as the decrease of transient voltage and luminescence without any loss in growth rate, emerges as a direct attempt to reduce the structural damage inflicted by plasma discharges [30,31]. A fundamental understanding in both theoretical and phenomenological level, however, is sparsely available to date. Massive-scale production of inorganic layer is also limited by the fact that soft plasma regime is reported to be initiated above ~20 min at least, considering that its activation is known to be thickness-dependent [31,32]. Nevertheless, the restraint on vigorous plasma activities results in a simultaneous improvement in thickness and compactness of inorganic layer. On the contrary, inorganic compounds with structural defects can be utilized for developing functional materials due to large surface area [3–5,33]. For instance, ZnO/C composite with hierarchical porous structure was fabricated using ice and micelle templating [33]. Its photocatalytic activity was found to be 7 times greater than that without hierarchical pores due to a great number of electron paths available within the 3D architecture. By promoting the porosity level, Bayati group has developed inorganic layer via PE as a promising candidate for supporting solid-state catalysts [34–36]. The protective capabilities of these inorganic layers, however, is expected to be less desirable since corrosive medium would be able to penetrate the metal substrate with ease through interconnected micro pores. The plasma-induced defects work as high-energy sites beneficial for firm anchoring of catalyst particles. In addition, such micro-defects are also responsible for enhancing chemical responses of the inorganic layer itself such as in triggering the nucleation of organic clusters via self-assembly at room temperature [12,28]. Thus, the existence of micro defects in inorganic layer via PE raises a challenging trade-off in satisfying structural and functional properties together. A versatile modification on the

discharge characteristics is therefore required to control the formation of inorganic layer at a core level. In this work, we propose a strategy to utilize plasma discharges exhibiting soft characteristics that is activated from the onset of electrical breakdown (~20 s). This is achieved by changing the electrolyte chemistry at the anode-electrolyte interface with the addition of a suitable complexing agent working as electron donor that can trigger rapid formation of electrical double layer (EDL) after disruption by plasma activities. As a case study, Al 6061 alloy was subjected to PE in an electrolyte containing aminopolycarboxylic acids, which have a wide spectrum of denticity and binding affinity towards metal-derived surfaces due to different numbers of electron-rich groups such as -COOH and tertiary amine. The present study demonstrated that the selection of aminopolycarboxylic acid as complexing agent was highly influential to the porosity level of inorganic layer as well as the metastable transformation of constitutive compounds. Accordingly, the electrochemical mechanism of complexing agents was investigated theoretically via quantum chemical calculations and experimentally via impedance analysis. We found that complexing agents interact with inorganic surface by donating electron pairs, strengthening the construction of EDL and accelerate the recovery of EDL. This would maintain a homogeneous electrical field that prevents localization of breakdown spots, resulting in a restraint on the destructive effect of plasma discharges. Hence, anti-corrosion performance of inorganic layer formed in electrolyte containing complexing agent with high denticity was found to be three orders of magnitude greater than that formed with simple complexing agent. Interestingly, such compact layer exhibited a catalytic activity in excess of 70% in ambient condition owing to the irregular surface caused by soft plasma. With a nearly defect-free inorganic layer supporting such responsive surface, our study proposed the feasible pathway to achieve structural and functional properties together using plasma treatment method in liquid electrolyte, which would be suitable for massive scale industries.

2. Experimental 2.1. Fabrication of inorganic layer Al 6061 alloy coupons with a chemical composition of 1.1 Mg, 0.71 Si, 0.5 Fe, 0.24 Cu, 0.12 Mn, 0.05 Zn, 0.05 Ti, and balance Al (in wt.%) were ground mechanically and cleaned ultrasonically in ethanol before further processing. Al 6061 alloy was taken as the substrate of interest as the oxides of Al are beneficial for multiple applications [37,38]. Details of experimental conditions were available elsewhere [10,12]. In short, an electrolytic cell was constructed by setting Al alloy as the anode and stainless steel net as cathode in a glass vessel, which was equipped with stirring and cooling systems to maintain the temperature around 288 K. PE treatments were performed for 300 s using 60 Hz AC signal at a current density of 100 mA cm-2. The electrolytes were formulated from KOH (0.11 M), Na2SiO3 (0.033 M) and an aminopolycarboxylic acid (0.025 M). Three complexing agents were selected: nitrilotriacetic acid (NTA; N(CH3COOH)3; tetradentate), ethylenediaminetetraacetic acid (EDTA; C2N2(CH3COOH)4; hexadentate), and diethylenetriaminepentaacetic acid (DTPA; C4N3(CH3COOH)5; octadentate). The changes in visual appearance of plasma discharges during PE were captured digitally (digital single-lens reflex camera, Canon EOS700D). From these photos, the size and population of plasma sparks were evaluated with an aid of an image analysis software (Image Analyzer 1.33) and the result was described statistically (Origin 2018).

2.2. Characterizations on microstructure and composition The morphologies of inorganic layers formed in electrolytes containing NTA, EDTA, and DTPA (IL-NTA, IL-EDTA, and IL-DTPA, respectively) were observed using a fieldemission scanning electron microscope (FE-SEM, Hitachi S-4800) coupled with energydispersive X-ray spectroscopy (EDS, Horiba) for elemental analysis. Surface porosity of

inorganic layers was estimated based on SEM images taken from ten different areas for each condition. The surface roughness of inorganic layer was evaluated from 3D surface profile data (3D surface profiler, ContourGT-K 3D). Field-emission transmission electron microscope (FE-TEM, Technai F20) was utilized at 200 kV to perform in-depth observation on inorganic layer and to obtain selected area electron diffraction (SAED) pattern. The specimens for TEM observation were obtained using focused ion beam milling (FIB, Helios Nanolab 600). Constitutive compounds were determined based on X-ray diffraction pattern (XRD, Rigaku D-MAX 2500) using Cu Kα radiation. The oxidation states of inorganic layers were evaluated via X-ray photoelectron spectroscopy (XPS, VG Microtech ESCA 2000).

2.3. Electrochemical measurements Electrochemical impedance spectroscopy (EIS) measurements were performed from 106 to 0.1 Hz at 10 points per decade using 10 V rms AC signal (potentiostat, Gamry Interface 1000). Potentiodynamic polarization scans were conducted from -0.2 to 0.4 V with respect to open circuit potential at a constant rate of 1 mV s-1. Both techniques were carried out with a potentiostat with a three-electrode setting: inorganic layer with an exposed area of 1 cm2 as working electrode, a platinum plate as counter electrode, and Ag/AgCl as reference electrode. Electrochemical measurements were repeated at least 3 times to gain sufficient reliability. Electrochemical mechanism of aminopolycarboxylic acids during PE was investigated in KOH (0.11 M), Na2SiO3 (0.033 M) and aminopolycarboxylic acid (0.025 M). An aqueous solution of NaCl (3.5 wt.%) was employed to reveal the chemical stability of inorganic layer.

2.4. Catalytic activity The catalytic activity of inorganic layer was evaluated based on the degradation of methylene blue (MB) via peroxidation route. The initial dye solution contained 6.25 mmol L-1 MB and

135 mmol L-1 H2O2 at natural pH. Inorganic layers with a surface area of 450 mm2 were immersed in 10 ml dye solution at room temperature for 48 h. The samples were recycled five times to evaluate the stability in catalytic performance. After each experiment, inorganic layer was cleaned with ethanol and dried with warm air before re-immersion into fresh MB solution. The concentration of MB post-decomposition was determined by comparing its light absorbance (UV-vis spectrometer, Varian Cary 5000 Scan) with MB solutions of known concentration at γmax = 665 nm. After catalytic treatment, MB solution was characterized utilizing Fourier transform infrared spectroscope (FTIR, Perkin Elmer Spectrum 100) and gas-chromatography coupled with mass spectroscope (GC-MS, Shimadzu GCMS-QP2010 Ultra) in order to identify the degradation mechanism and intermediate products.

2.5. Quantum chemical analysis Calculations of frontier molecular orbitals were performed using Gaussian 09W package using density functional theory (DFT) at B3LYP level with 6-31G++(d,p) basis set. The polarizable continuum model was adapted in order to account for the effects of aqueous solvation implicitly. Optimization of molecular structures was verified by calculating their frequencies at the same level of theory. All computations were performed on aminopolycarboxylic acids in their deprotonated states suitable for representing the effects of electrolyte system with high alkalinity. We calculated the energies of frontier molecular orbitals, such as highest occupied molecular orbital (HOMO) as well as lowest unoccupied molecular orbital (LUMO), and derived them into other quantum chemical descriptors such as the ionization potential (I) and electron affinity (A) are defined by Koopmans theorem with respect to orbital energies [39].

I = - EHOMO (1)

A = ELUMO

(2)

These fundamental characteristics also define the Mulliken electronegativity (χ) and absolute hardness (η).

χ =

I + A 2

η =

I - A 2

(3)

(4) The donor-acceptor interaction between aminopolycarboxylic acid molecules and inorganic layer surface is then evaluated by calculating the fraction of electrons (∆N) involved.

∆N =

χAl - χmol 2 (ηAl + ηmol)

(5)

Our calculation of ∆N considered theoretical values of χAl (3.23 eV mol-1) and ηAl (2.77 eV mol-1), as suggested by Pearson [40].

3. Results 3.1. Transient responses during the formation of inorganic layer During PE treatment in galvanostatic mode, the cell voltage (rms) changes over time due to the increase in thickness of inorganic layer and other electrochemical processes accompanying its formation. Fig. 1 shows transient characteristics of PE using alkalinesilicate electrolyte containing different complexing agents. The present electrolysis consists of three stages whose cell voltages intensify at different rates (Fig. 1a), each with a distinctive appearance of plasma discharges (insets). Stage I shows a sharp increase in cell voltage due to a surge of electrical resistance taking place when the surface of metal anode transforms into an insulating passive film. The increase in voltage continues briefly until the breakdown voltage at which the electrical field is sufficient to break through the resistance of passive film, causing an electrical breakdown [6,7] that indicate the start of stage II at ~20 s. The

breakdown voltages are in the order of NTA (~280 V), EDTA (~320 V), and DTPA (~345 V), which are proportional to the thickness of their respective passive films. The dielectric breakdown triggers instantaneously an avalanche of high energy electrons accompanying numerous plasma discharges throughout the anode surface [7,20,24]. This leads to a transition in growth mechanism in stage II, from conventional chemical reactions following the linear relationship of Ohm’s law, to plasma-driven electrochemical processes [7,16,18]. Hence, the increment of cell voltage in stage II differs significantly than that in stage I. Entering stage III, the growth of inorganic layer relies practically on the influence of plasma sparks and the increase of cell voltage is attributed mainly to the breakdown phenomenon instead of the resistance of inorganic layer [41]. The characteristics of short-lived sparks on the anode surface changes as a function of time. Fig. 1b shows that at the transition between stage I and II, plasma discharges in EDTA- and DTPA-containing electrolytes have already differentiated into various sizes while those with NTA remain in similar radius, suggesting that surface reformation of Al 6061 alloy in NTAcontaining electrolyte prior to breakdown is less advanced. In stage II and III, discharge sites in NTA, EDTA, and DTPA electrolytes appear to be different in size (Figs. 1c, d). This is because clusters of plasma discharges tend to appear consecutively in a ‘cascade’, the intensity of which is proportional to the number of active discharges at given area [24]. The transient characteristics of discharge cascades, such as distribution, brightness, size and population, are determined mainly by the local microstructural profile. In NTA electrolyte, for instance, numerous dim cascades are localized around a bright center. This is unlike those in EDTA and DTPA electrolytes, where spherical sparks exhibiting moderate intensities appear throughout the surface. Furthermore, Fig.1e shows that plasma discharges in electrolyte containing EDTA and DTPA are more populated that containing NTA, which confirms that the generation discharge events takes place more homogeneously

in electrolytes containing aminopolycarboxylic acid having large denticity.

3.2. Microstructural interpretation of inorganic layer The microstructure of inorganic layer can be considered ‘fingerprints’ of plasma arcs since they would deform the inorganic layer according to the distribution of active discharge. Fig. 2 displays the microstructures and surface topography of inorganic layers after PE treatment for 300 s. It is apparent from Fig. 2a-c that inorganic layers show different degrees of porosity with respect to the denticity of complexing agent in the electrolyte. The open porosities of ILNTA, IL-EDTA, and IL-EDTA are measured to be 10.5%, 3.2%, and 1.7%, respectively. A number of micro pores in IL-NTA are interconnected in an elongated shape parallel to the surface as a result of discharge cascade intersecting one another. The presence of these ‘cascade channels’ whose diameter around 2-4 μm suggests that prolonged sequences of discharge cascade are dominant in the case with NTA. In contrast, IL-EDTA and IL-DTPA show micro pores with a circular shape. In some cases, micro defects in IL-EDTA are likely to overlap such that large micro pores above ~5 μm are present. These large cavities, interestingly, are absent in IL-DTPA even though the population of plasma discharges during PE in DTPA electrolyte is apparently larger than the cases with NTA or EDTA (Fig. 1e). Furthermore, Figs. 2d-f show that the average thicknesses of three different inorganic layers are calculated to be 2.5 ± 0.8 μm, 3.8 ± 0.5 μm, and 5.5 ± 0.4 μm for IL-NTA, IL-EDTA, and IL-DTPA, respectively. All of the inorganic layers consist of a porous outer layer exposed to the environment and a compact inner layer close to the metal substrate. IL-NTA suffers from a severe fluctuation in thickness as well as the presence of deep cascade channels that can expose metal substrate to potential corrosive agents with ease. Cross-sectional micrographs of IL-EDTA and IL-DTPA are consistent with the surface appearance, confirming that they are more compact than IL-NTA. A number of ‘discharge channels’ below ~1 μm wide, which is

known to be remnants of volcanic-type discharges [20,21,24], are still visible in IL-EDTA. 3D visualizations of inorganic layers shown in Fig. 2g-i show topographical variations as a result of plasma activities. The large craters and channels appear presumably due to localization of numerous plasma discharges, which is evident particularly in IL-NTA. On the other hand, IL-DTPA is compact throughout the thickness (Fig. 2f). Its surface topography features a large number of slight fluctuations as residual tracks of numerous plasma discharges. Accordingly, the values of surface roughness (Ra) are in the order of IL-NTA (1.72 μm), IL-EDTA (1.35 μm), and IL-DTPA (1.00 μm), indicating the inverse relationship between Ra and the denticity of complexing agent in PE electrolyte. In normal, intensive plasma discharges would result apparently in high porosity that can be observed from top or side view of inorganic layer. Hence, the thickness and compactness of inorganic layer fabricated in the presence of aminopolycarboxylic acid with large denticity seem counterintuitive to the fact that large population of plasma discharges are pointed out during PE. This implies that the addition of such complexing agents can result in soft plasma discharges, which represent less vigorous plasma activities causing minimum structural damage without an expense of material growth [10,30,31]. Unlike soft plasma regime generated by subjecting the anode to polarization using a specific waveform [30,31], the present approach relies on electrochemical effects on aminopolycarboxylic acid to control the destructive effect of plasma discharges.

3.3. Chemical transformation of inorganic layer First, elemental analyses via EDS were performed on two areas of interest based on SEM micrographs in Fig. 2a-c: dark area with a relatively flat topography and bright area having a coral-like morphology. Fig. 3a suggests based on the atomic ratio between Al and Si that dark area consists mainly of Al-compounds containing Si impurities while bright area contains a

considerable amount of Si-compounds derived from silicate electrolyte. Most notably, Si-rich areas in IL-DTPA show an Al:Si ratio of ~2.94, strikingly close to that in Al6Si2O13 that is known for a superior physical and chemical stabilities among aluminosilicates [42,43]. Subsequently, the constitutive compounds of inorganic layers were determined based on XRD analysis, as displayed in Fig. 3b. These patterns show that oxidation of Al substrate under the present conditions produces mainly γ-Al2O3, which forms through rapid cooling when molten material is quenched upon contact with the cool electrolyte. The peaks of Al(OH)3 in IL-NTA suggest that the thermal energy provided by plasma discharges is insufficient for a complete conversion from Al(OH)3 to γ-Al2O3. When complexing agents with larger denticity are employed, on the other hand, dehydration of Al(OH)3 is fully achieved due to an easy access to thermal energy from the abundance of plasma discharges. Accordingly, the peaks of Al6Si2O13 are more pronounced in IL-DTPA than in IL-EDTA as an intensive heating by plasma discharges favors the solid-state reaction between γ-Al2O3, which is the major constituent of inorganic layer, and SiO2, which is a result chemical incorporation of electrolyte component through plasma activities. In order to verify the transformation processes during the formation of inorganic layer, microstructural observation was performed in high-resolution via TEM with a specific focus on the surface area where inorganic layer, high-energy plasma, and cool electrolyte coexisted during PE. Fig. 3c shows that the IL-NTA consists of nanocrystalline regions within amorphous matrix while a layer of fully amorphous material is detected in the outermost surface. Incomplete transformation from Al(OH)3 to γ-Al2O3 is verified, as well as partial crystallization of SiO2 derived from the amorphous layer. Thus, γ-Al2O3, SiO2, and Al(OH)3 are detected in SAED pattern of IL-NTA (Fig. 3d). On the other hand, Fig. 3e displays a number of nanocrystalline grains whose diameter around 5-8 nm in IL-EDTA. Transformation from γ-Al2O3 to Al6Si2O13 was observed to be at

the expense of amorphous SiO2 that extends from the outermost layer. Accordingly, SAED pattern of IL-EDTA (Fig. 3f) is different from that of IL-NTA owing to the existence of Al6Si2O13. In the event where heat flux from plasma discharges is insufficient for solid-state diffusion of SiO2 towards γ-Al2O3, crystallization of SiO2 takes place instead. Moreover, a large population of plasma discharges in the formation of IL-DTPA provides larger heat input favorable for high-temperature transformation processes. It is, thus, Fig. 3g shows a relatively large nanocrystalline grains up to ~15 nm in diameter, which consist of γ-Al2O3 mainly and Al6Si2O13 particularly near the surface. Crystalline SiO2 was less apparent, suggesting that plasma heating accelerates the reaction between γ-Al2O3 and amorphous SiO2 such that crystallization of SiO2 is less favored. This is supported by SAED pattern of IL-DTPA displayed in Fig. 3h, which agrees well with macro-scale XRD responses in Fig. 3b. Analysis of XPS spectra was performed to quantify the constitutive compounds based on their oxidation states such that amorphous components are taken into account. Generally, high-resolution spectra of Al 2p (Fig. 4a-d) and Si 2p (Fig. 4e-h) display a substantial shift of binding energy between IL-NTA, IL-EDTA, and IL-DTPA. The positive shift of Al 2p peak in IL-DTPA is attributed to the dominant presence of Al6Si2O13, which exhibit a binding energy of ~74.8 eV [44] that is higher than those associated with γ-Al2O3 (~73.8 eV) and Al(OH)3 (~74.2 eV) [45]. A similar shift is also observed for Si 2p spectra since Si atoms in Al6Si2O13 exhibited stronger binding energy than SiO2, i.e., ~103.2 eV [44] and ~102.5 eV [46], respectively. Deconvolution treatments are conducted on XPS spectra in Figs. 4a and e for Al 2p and Si 2p, respectively and the relative fractions (Xi) of each component are evaluated by the calculating contribution of the corresponding peak (Ii) with respect to the total peak area (Itotal) of a given element.

Xi =

Ii Itotal

(6)

Table 1 reveals that Al6Si2O13 constitutes 0%, 30%, and 62% of Al-derived compounds in ILNTA, IL-EDTA, and IL-DTPA, respectively. Similarly, Table 2 estimates that ~36% of Si content in IL-EDTA reacts with γ-Al2O3 to produce Al6Si2O13, whereas IL-DTPA constitutes ~47% of its Si content in the form of Al6Si2O13. The chemical analyses above suggest that the use of an aminopolycarboxylic acid with large denticity would provide sufficient energy to trigger the formation of Al6Si2O13 as a result of numerous discharge events, considering that the formation of Al6Si2O13 requires a minimum temperature of ~1193 K, which is the highest among aluminosilicate compounds [42,43]. The presence of Al6Si2O13 has been widely reported to improve the resistance of Al2O3-based inorganic layers against chemical and thermal attacks[10,43]. Hence, protective performance would be enhanced by selecting aminopolycarboxylic acid with high denticity. The selection of aminopolycarboxylic acid with low denticity, on the other hand, favors the formation of γAl2O3 and Al(OH)3, which are widely used in niche applications due to their relative ease of interaction with other substances [3,4,47,48]. These compounds are likely to synergize well with the porous architecture of inorganic layers generated by PE in order to functionalize them for a variety of potential applications.

3.4. Protection performance of inorganic layer against corrosive medium The anti-corrosion performance of inorganic layers were evaluated by potentiodynamic polarization test in 3.5 wt.% NaCl solution after short-term (1 h) and long-term (24 h) exposures, and the current-potential responses were compared to that of bare Al 6061 alloy in Fig. 5a. Table 3 lists all electrochemical parameters obtained by analyzing polarization curves by Tafel extrapolation. The general durability of inorganic layers in such aggressive medium

was compared based on their magnitude of polarization resistance (Rp) calculated using Stern-Geary equation [49],

Rp =

β a × | βc| 2.303 icorr ( β a + | βc|)

(7)

where icorr, βa, and βc is the corrosion current density, slopes of anodic and cathodic branches of polarization curves, respectively. Irrespective of immersion time in aggressive solution, polarization curves shift to the low region of icorr in the order of bare Al 6061 alloy, IL-NTA, IL-EDTA, and IL-DTPA, indicating a decrease in corrosion kinetics. This would be attributed to the combined influences of two aspects: the thickness of inorganic layers and the compactness of their microstructure. Corrosion properties is mainly determined by how well a protective coating separates the corrosive ions from an underlying material, which would be best represented by the average thickness of coating layer. For metal substrate, however, the shielding capability of a coating might only be as strong as its weakest point. Once corrosive ions penetrated the coating successfully, metal degradation would be imminent even though the coating itself is still intact. In addition, the presence of chemically active components such as Al(OH)3 would reduce the corrosion potential (Ecorr) indicative of a vulnerability to chemical attacks. Based on the values of Rp after immersion in 3.5 wt.% NaCl solution for 1 h, the denticity of complexing agent in PE electrolyte is found to be proportional to the protective capability of the resultant inorganic layer. This is because the compact structure of IL-DTPA would inhibit the corrosion process more significantly than the porous architecture of IL-EDTA or ILNTA. For a short period, the natural passive film on Al 6061 is somewhat protective against corrosive ions. Nevertheless, the Rp of the present inorganic layers can provide greater anticorrosion protection up to ~270 times greater than that offered by the passive film on Al 6061 alloy. After 24 h, the corrosion behavior of bare Al 6061 alloy has deteriorated by two orders

of magnitude due to the chemical decay of passive film, which was inferred by the susceptibility to pitting corrosion, as indicated by red circles in Fig. 5a. This breakdown of protective layer is also found in IL-NTA after 24 h considering the presence of cascade channels working as weak points in preserving the physical integrity of inorganic layer. On the contrary, IL-DTPA is capable of maintaining the value of Rp in the same order even after 24 h of exposure while the corrosion performance IL-EDTA and IL-NTA each have declined by more than one order of magnitude. Further insight into the mechanism of electrochemical protection can be obtained by analyzing EIS data obtained in 3.5 wt.% NaCl solution. The Nyquist plots in Fig. 5b demonstrate typical resistive-capacitive (R-C) responses that represent inorganic layers on top of a conductive electrode. Evidently, IL-DTPA demonstrates larger impedance modulus as compared to those exhibited by IL-EDTA and IL-NTA, which suggest a sluggish dissolution kinetics of the protective layer. To decouple different electrochemical processes taking place during immersion in corrosive medium, the impedance response is simulated to an equivalent circuit model by taking microstructural characteristics into consideration. The capacitive part of total impedance (Ztotal) is represented by constant phase element (CPE) rather than by regular capacitor, considering the distribution of capacitance resulted from the surface motifs of inorganic layer [9–11,50]. A CPE is defined by ZCPE = (Y(jω)n)-1, where ZCPE is the impedance of a CPE, j is the imaginary number, ω is the angular frequency, Y and n are the characteristic parameters of CPE. The total impedance of an R-C response is therefore Ztotal = R-1 + ZCPE-1. Depending upon the microstructural complexity, the impedance responses of inorganic layers would be interpreted into two or three phenomena, all of which are quantified in Table 4. Three different R-C responses are identified in IL-NTA and IL-EDTA since the connected micro pores at their surfaces give rise to severe fluctuation in the bulk electrochemical

responses while two R-C responses are detected in IL-DTPA due to the compact structure consisting only outer and inner layer. In these circuit models, Rs is the electrolyte resistance, Rsurf and Qsurf are capacitive and resistive characteristics of highly porous surfaces of IL-NTA and IL-EDTA, Rout and Qout are associated with outer layer while Rin and Qin represent the responses of inner layer. Regardless of the microstructure, Rin is substantially larger than either Rsurf or Rout, indicating that inner layer is the main component providing electrochemical protection. Consequently, IL-NTA would be less protective as indicated by a low value of Rin. While IL-EDTA exhibits an intermediate level of Rin, the porous surface is rather reactive to chemical disruptions. Ultimately, the fact that Rin of IL-DTPA is greater by over two orders of magnitude than IL-NTA demonstrates reliable electrochemical protection.

3.5. Catalytic activity of inorganic layer for waste degradation One of the potential applications of alumina-based materials is the catalytic treatment of waste water by a synergistic action with H2O2 [3,4,47]. To evaluate the catalytic properties of the present inorganic layers, we immersed IL-NTA, IL-EDTA, and IL-DTPA in aqueous solution containing MB, which represents typical organic contaminants in water, in the presence of H2O2 as the initiator. Figs. 5c-e shows the chronological degradation of MB up to 48 h with an aid of IL-NTA, IL-EDTA, and IL-DTPA, respectively, and the changes in physical appearance of the sample solution. Regardless of the microstructure and constituent of inorganic layers, severe decomposition of MB takes place in the first ~3 h, during which IL-NTA is able to degrade MB to a greater extent than either IL-EDTA or IL-DTPA. The catalytic activity starts to saturate from ~24 h, whereby the absorption characteristics of MB solution has collapsed entirely in the case of IL-NTA. The cases with IL-EDTA and ILDTPA, on the other hand, show evident peak at 665 nm after 48 h despite a notable decrease in their intensities.

Fig.S1 shows a linear relationship between the light absorbance and concentration of MB solution as the basis to calculate the change in its concentration over time. The catalytic efficiency (η) by inorganic layer would be defined as,

η =

C0 - Ct C0

× 100%

(8)

where C0 is initial concentration of MB (6.25 mmol L-1) and Ct is the concentration of MB at a given time during catalytic treatment. Fig. 5f displays the η values of the present samples evaluated until 48 h, which are compared to that of inorganic layer fabricated without the presence of complexing agents (IL). Typical microstructure and composition of IL are reported previously [7,10,19,30]. It is evident that over 50% of MB content has already decomposed within 3 h when IL-NTA or IL-EDTA are employed as catalyst while the decomposition by IL and IL-DTPA is somewhat delayed. This is attributed mainly to the prevalence of Al(OH)3 and γ-Al2O3 in IL-NTA and IL-EDTA, respectively. After 48 h, the final values of η in the cases of IL-EDTA and IL-DTPA are found to be ~77% and ~75%, respectively, while IL-NTA displays a greater magnitude of η in excess of ~84% such that the bluish shade of MB solution is eliminated completely (Fig 5f insets). On the other hand, IL causes a sluggish decomposition of MB that reaches a low plateau at 52.5%. This significant difference is stemming from the absence of soft plasma discharges in the fabrication of IL. With an addition of aminopolycarboxylic acid, the mild intensity of soft plasma discharges contributes to the formation of metastable phases suitable for catalytic applications. The difference between inorganic layers in terms of catalytic performance would be further exemplified by looking into their stability over multiple cycles, as displayed in Fig. 5g. Inorganic layers fabricated with soft plasma discharges exhibit excellent long-term catalytic stability over 5 catalysis cycles, with a minor decline in η with IL-NTA (~5%) and IL-DTPA (~7%) while the value of η with IL-DTPA decreases more evidently (~13%). This is because

shallow grooves in the topography of IL-DTPA may cause early saturation of MB at the surface, which is thought to be responsible for such early deterioration of η. On the other hand, the η value of IL declines beyond half of its initial magnitude, which can be attributed to the presence of α-Al2O3 commonly found in inorganic layers fabricated by PE in the absence of soft plasma [7,10,29,36]. The rhombohedral structure of α-Al2O3 shows a compact arrangement of ions, which minimizes the adsorbability of MB necessary for a successful catalysis. A number of studies have confirmed experimentally the catalytic activity of alumina-based inorganic layers towards organic dyes, but a conclusive mechanism remained veiled to date [47,51]. It is unlikely that well-known mechanisms like photo-excitation [15,16] or Fentontype reactions [4,52] would be operative in this case. To shed more light into the oxidative degradation of MB in the present study, MB solutions before and after treatment with ILNTA were characterized with FTIR and GC-MS. Fig. S2 denotes the change in functional groups after 48 h of catalytic incubation, from which the peak of C=N at ~1600 cm-1 is annihilated. In addition, the peak at ~3320 assigned for -OH is widened to be around 3355 3245 cm-1 due to hydrogen bonding with N-H groups in decomposition products. A new peak for S=O (~1070 cm-1) also appears after catalysis with the present samples. Those changes suggest that the heteronucleus ring at the center of MB molecule, which contains main chromophores, breaks into smaller fragments. Decomposition of organic dyes like MB is commonly associated with the presence of active radicals that trigger the oxidative degradation of MB molecules [47,52,53]. In aqueous solution, a large number of -OH groups decorate the outermost surface of inorganic layers. This condition allows the adsorption of MB molecules on the inorganic surface easily via sulfur atom while H2O2 molecules attach to the surface via hydrogen bonds, which facilitated chemical reactions between them. Thus, a number of studies reported that γ-Al2O3 itself

exhibits catalytic responses towards chemical change of -OH containing groups such as ethanol and phenol instead of working passively as catalyst support [3,4,48,54]. It is likely that the interaction between oxygen atoms in H2O2 and acidic sites in Al2O3 or Al(OH)3 favors homolytic cleavage of H2O2 into ·OH radicals necessary for the degradation of MB molecules [47]. The identification of degradation products identified by GC-MS (Fig S3a-e) suggests that the mechanism of catalysis in the present study is in a good agreement with the cases involving ·OH radicals [47,48,52,53]. Taking both the FTIR and GC-MS data into consideration, a possible mechanism of MB degradation is proposed in Fig. 5h. A successful decomposition of MB is indicated by the absence of the peak at m/z value of 284. Heteroaromatic ring in MB molecules would likely to be the most vulnerable due to the electrophilic sites in C=N and C=S bonds. The breakdown of this central ring produces intermediate products containing sulfoxide and hydroxyl groups such as those with m/z values of 200 and 125, which are attributed to the presence of C8H12N2SO2, and C6H7NO2, respectively as shown in Figs. S3ab. Further decomposition rests mainly on the release of sulfoxide and amine groups, resulting in simpler aromatic compounds having m/z values of 174, 152, and 120, which are assigned to C6H6SO3, C8H12N2O, and C4H8O4, respectively as displayed in Figs. S3c-e. These compounds would undergo mineralization into inorganic substances with low molecular weight such as CO2, H2O and a variety of inorganic ions. The surface of IL-NTA is decorated with a larger population of -OH groups than that of ILEDTA and IL-DTPA owing to a large surface area as well as the presence of Al(OH)3. Hence, IL-NTA promotes the formation of radicals more effectively, resulting in the superior catalytic properties. It is interesting to note that, while the microstructures of IL-EDTA and IL-DTPA are suitable to enhance physical and chemical shielding against an aggressive environment, they also reach a notable η above 75%. The ·OH radicals generated in those

cases are proven to be sufficient to decompose MB for long-term treatment due to their natural curvature as the residual tracks of soft plasma.

4. Discussion 4.1. Electrochemical roles of complexing agents Few reports [10,55,56] have documented the electrochemical behavior of plasma-inspired techniques involving EDTA specifically but the general mechanism of complexing agents in altering plasma characteristics remain unresolved. To this end, the interaction between aminopolycarboxylic acids and inorganic layer was investigated by performing theoretical calculations based on DFT principles. Fig. 6a displays the distribution of frontier molecular orbitals in NTA, EDTA, and DTPA under aqueous solvation as their respective ions: NTA3-, EDTA4-, and DTPA5- since each -COOH group loses an acidic H+ in the alkaline electrolyte (pH ~13). Table 5 displays the energies of frontier orbitals, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), as well as other descriptors necessary to understand the interaction of complexing agents with Al-based inorganic layers. A molecule with a high EHOMO would be able to donate electron easily to the electrophilic metal orbitals, leading to stronger adsorption on metal-derived surfaces while ELUMO represents the tendency of the molecule to accept electrons from stronger electrondonating species [12,28]. In addition, the gap between EHOMO and ELUMO (ΔE) is a general measure of chemical reactivity. A large magnitude of ΔE suggests that the adsorption layer containing a given molecule would favor the interfacial reactions like hydration and dissolution. Accordingly, a relatively wide energy gap of NTA (~6.15 eV) would be responsible for the presence of Al(OH)3 because the γ-Al2O3 at the surface of IL-NTA would be hydrated with ease. During physisorption or chemisorption, most complexing agents would likely to work as

electron donor while metal-containing surface would be the acceptor. The stability of such donor-acceptor interaction was evaluated based on the fraction of electron transferred (∆N). Table 5 indicates that the values of ∆N show two-fold increments roughly when the denticity of complex ions increases by two units. Therefore, the adsorption of DTPA would be more favorable than NTA and EDTA due to a higher magnitude of ∆N, which can be attributed to two different factors. First, DTPA molecule is decorated with a sheer number of functional groups with electron-donating tendency such as five -COOH groups and two tertiary amines where HOMO is concentrated (Fig. 6a). Second, a higher dipole moment (μ) of DTPA than NTA or EDTA implies that the electron clouds would be easily deformable in the presence of an electrical field, assisting the adsorption to the surface of inorganic layer during PE.

4.2. Characteristics of plasma discharges by considering roles of complexing agents The generation of plasma discharges is triggered on the interface between inorganic layer and electrolyte where organic-inorganic interaction is most influential [20,21]. We, therefore, conducted a set of experiments using EIS to reveal the interfacial processes in PE electrolytes containing three different complexing agents, which is summarized in Fig. 6c. The analyses of electrochemical responses as well as the impedance parameters drawn from a successful fitting are elucidated in Supplementary Notes and Table S1 (See Supplementary Information). Briefly, the electrochemical behavior of inorganic layer during its growth can be discussed in its entirety by focusing on pits of inorganic layer (Fig. 6c inset). The electrochemical responses in such cavity are attributed to: (i) changes in the surface of inorganic layer, including the formation of EDL (Rct and Qdl); (ii) voltage drops in inorganic layer itself (Ril and Qil); and (iii) voltage drops triggered at inorganic layer-substrate interface (Rint and Qint), all of which are illustrated in Fig. 6c. Each of these electrochemical responses occurs repeatedly in a specific time interval

commonly referred to as time constant (τ), the calculation of which is described in Supplementary Note. Impedance parameters of at the surface of inorganic layer shown in Table 6 reveals that EDL forms more quickly in the presence of DTPA since the corresponding value of τ (τdl) is calculated to be 1.2 μs, which is significantly less than those form in the presence of NTA (17.2 μs) and EDTA (6.8 μs). With DTPA, EDL is stabilized by the strong donor-acceptor interaction with inorganic layer. EDTA and NTA, on the other hand, attach loosely to inorganic layer such that EDL requires more time to reform between anodic polarizations. A rapid formation of EDL in the presence of a complexing agent would be associated with the fast recovery of a homogeneous electrical field after severe disruptions such as reversepolarization of alternating current and topographical reformation by plasma discharges. As a comparison, the formation of EDL during PE in a typical alkaline-silicate electrolyte without complexing agents was estimated on the basis of ionic movements to be 175-260 μs [57], which is significantly larger than the formation rate of EDL calculated in this study. Our microstructural evidence (Fig. 2) supports that such phenomena would prevent localization of plasma discharges such that their destructive effect would be governed. Furthermore, as the presence of EDL is central to the initiation of plasma activities, the donor-acceptor interaction of complexing agent is expected to set off more discharge events homogeneously, as confirmed in Fig. 1a (inset) and Fig 1e. Interfacial modification by complexing agent also affects the change in energy distribution. Throughout PE, a fraction of energy input diverts incidentally to the electrical charging phenomena of inorganic layer, the energy of which (ECh) is estimated by the following equation [58],

ECh =

U2CT 2

-t

(1 - e ) (C

T Rs

)

2

(9)

where U is potential, CT is total capacitance, t is time, and Rs is electrolyte resistance. A large magnitude of ECh indicates that the energy input will be spent less effectively to maintain breakdown phenomena that generate plasma discharges. In a common electrolyte, ECh would be inversely proportional with the thickness of EDL (λD) estimated by Debye-Hückel relation,

λD =

(

ε0εrwNAkbT 2

2F I

)

1 2

(10)

where ε0 and εrw are the permittivity of vacuum and the dielectric constant of water, respectively. NA is Avogadro’s number, F is Faraday’s constant, kb is Boltzmann’s constant, T is the electrolyte temperature which is maintained at 288 K, and I is the ionic strength of the electrolyte defined as

I =

1 n ∑ c z2 2 i=1 i i

(11)

where ci and zi are the concentration and valence of individual ions. Accordingly, λD in electrolytes containing NTA, EDTA, and DTPA are calculated to be ~19 nm, ~16 nm, ~14 nm, respectively while λD of the silicate electrolyte without complexing agent is ~25 nm. Instead of pure electrostatic interaction, lone electron pairs of complex ions are being donated to Al atoms, as illustrated in Fig. 6c, which would alter the chemistry of EDL in the present condition. Capacitive charging of inorganic layer is calculated from the total effective capacitance (CT) calculated from CPE parameters in real-time electrolyte system (See Supplementary Notes and Table S2). The values of CT, which consists of capacitances of double layer (Cdl) and inorganic layer (Cil), is determined to be ~1.07 nF, ~0.71 nF, and ~0.16 nF in electrolytes containing NTA, EDTA, and DTPA, respectively. A low CT of DTPA electrolyte implies that the energy input is allocated more effectively to generate plasma discharges, as confirmed experimentally by the high population of plasma discharges (Fig. 1e) and the superior thickness of inorganic layer (Fig. 2f). A combination of

chemical reinforcement of EDL and low electrical charging would be responsible for the soft attribute of plasma discharges which minimizes the micro defects while maintains the growth rate of inorganic layer.

4.3. Microstructural control to achieve chemical stability and catalytic activity Figure 6d outlines the reconstruction of EDL and its effects on the generation of soft plasma discharges underlying the formation of inorganic layers. First, it is noted that the variations in discharge characteristics would govern the microstructure of inorganic layer in both constructive and destructive manners. Intensive plasma sparks rupture EDL and thus disable the shielding against high electrical field on breakdown sites [32,57]. This would lead to further localization of plasma discharges over time, which amplifies their destructive effect on the microstructure [7,30,31]. Our study has provided theoretical and experimental evidences supporting that complexing agent will be able to accelerate the reformation of EDL. As represented in the top panel of Fig. 6d, the presence of complexing agent with larger denticity like DTPA gives rise to a faster rearrangement of EDL structure as compared to the low-denticity counterparts. Accordingly, the shielding effect of EDL will be restored rapidly, leading to the inhibition of a superfluous localization of discharge cascades. The growth of inorganic layer during PE relies mainly on the rheological flow of molten materials similar to volcanic activities. As described in the bottom panel of Fig. 6d, the growth of inorganic layer less shielded with EDL, such as that in NTA-based electrolyte, would rely on extensive plasma channels that consist of interconnecting discharges, the remnants of which are noted in Fig 2 as ‘cascade channel’ whose diameter is 2-4 μm. It is unlikely that these wide channels would be effective in facilitating the growth of inorganic layer effectively via melt-flow. When either EDTA or DTPA is involved, interestingly, the width of melt channel is presumably slimmer than that with NTA. A narrow passage we refer

to as ‘discharge channel' (Fig. 2e) would allow an efficient mass transfer due to an aid of capillary action. Supposedly, these channels consist of limited number of discharges, considering that their residual damage is less severe than those caused by cascade channels. As a result of different width of plasma channels, IL-EDTA and IL-DTPA are both thicker and more compact than IL-NTA, improving the anti-corrosion performance. This evidence suggested that the use of large complexing agents has beaten the trade-off between thickness and compactness of inorganic layer commonly reported in parametric studies on PE [6,7,18]. On the other hand, simple complexing agent like NTA is preferable for fabricating porous inorganic layer with large surface area. Transformation processes of metastable phases are also found to be incomplete because thermal distribution in localized discharges is less homogeneous. As a result, a combination of γ-Al2O3 and Al(OH)3 is obtained, and the catalytic affinity of such inorganic layer towards decomposition of organic dye is proven superior to its stable counterpart (Fig. 5c-g). Under such paradox between corrosion protection and catalytic activity, a question arises as to the feasibility of combining both structural reliabilities and functional properties together. After incubation for 24 h, Rp value of the present inorganic layers is in the range of 3.03 × 105 - 1.63 × 108 Ω cm2 (IL-NTA to IL-DTPA) while the value of η is approaching saturation between 74% - 82% (IL-DTPA to IL-NTA), which indicate that both protective and catalytic aspects are satisfied in the present work. Although the bulk structure of IL-DTPA is compact apparently (Fig. 2f), a large population of soft plasma discharges leads to an irregular topography (Fig, 2i). Such a surface contains high-energy sites preferable for catalyzing the homolytic cleavage of H2O2 into ·OH radicals. Thus, the present work reported for the first time a conformal inorganic layer where a combination of chemical stability and catalytic acitivity would be satisfied. Although the selection of low-weight complexing agent such as NTA is preferred in terms of catalytic activity, a large sacrifice in corrosion resistance would

restrict its usage in practical applications. In contrast, the presence of strong complexer as DTPA induces compact microstructure with dynamic surface, but long-term catalytic stability of the resultant inorganic layer is limited. Thus, the present study suggests that chemical stability and catalytic activity is optimized by utilizing alkaline silicate electrolyte with complexing agents having sufficiently large denticity like EDTA.

5. Conclusions Soft plasma discharges are generated by changing the chemistry of EDL upon addition of NTA, EDTA, or DTPA, which are beneficial for controlling the formation of Al-based inorganic layer exhibiting both anti-corrosion and catalytic properties. Inorganic layer fabricated in the presence of NTA comprises mainly γ-Al2O3 and Al(OH)3 with an apparent surface porosity of ~10.5%. When EDTA or DTPA is selected, the porosity of inorganic layers decreases to ~3.2% and ~1.7%, respectively, and stable constituent like Al6Si2O13 forms due to plasma-assisted heating. Complexing agents interact with inorganic surface by donating electron pairs, strengthening the construction and accelerate the recovery of EDL. This would maintain a homogeneous electrical field that restrains the destructive effect of plasma discharges by preventing localization of breakdown spots. As a result, soft plasma discharges are generated, resulting in a compact microstructure with dynamic surface topography that optimizes chemical stability and catalytic activity together.

Acknowledgements This research is supported by Research Program through the National Research Foundation, Republic of Korea (NRF-2017R1D1A1A0900021 and NRF-2019R1FA1062702). Y.G.K. acknowledges G.Y.H. for the fruitful discussion on structure analysis.

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Figures

Fig. 1. Transient characteristics of PE in electrolytes containing different complexing agents: (a) Cell voltage (rms) vs time curves of PE in silicate-based electrolytes containing NTA, EDTA, and DTPA, respectively. Three different stages are classified based on the chronological increase in cell voltage. Insets are representative display of plasma discharges at the end of stage I (~20 s), stage II (~60 s), and stage III (~300 s). Size distribution of plasma discharges at (b) 20 s, (c) 60 s, (d) 300 s. (e) Changes in the population of plasma discharges over time based on the fraction of the anode surface covered by active discharges. Dim discharge spots are omitted from calculations, as they are no longer considered active.

Fig. 2. Microstructural interpretation of inorganic layer formed by PE: Surface morphologies of (a) IL-NTA, (b) IL-EDTA, and (c) IL-DTPA after PE treatment for 300 s. Morphologies of (d) IL-NTA, (e) IL-EDTA, and (f) IL-DTPA taken from cross-section areas, comprising inner and outer layers. Cascade channels are traces of numerous discharges overlapping one another in a consecutive manner. Discharge channels are remnants of few eruptive discharges. 3D visualization of (g) IL-NTA, (h) IL-EDTA, and (i) IL-DTPA. Topographical variations appear due to the residual tracks of plasma discharges.

Fig. 3. Chemical transformation of inorganic layer formed by PE: (a) Al:Si ratio around dark area (shading fill) and bright area (solid fill) in IL-NTA, IL-EDTA, and IL-DTPA after PE treatment for 300 s. (b) Comparison of XRD patterns taken from IL-NTA, IL-EDTA, and ILDTPA after PE treatment for 300 s. Al peaks originate from the metal substrate. TEM images showing the surface of (c) IL-NTA, (e) IL-EDTA, and (g) IL-DTPA as well as HRTEM images describing the chemical transformations taking place in each inorganic layer during PE. Indexing of SAED patterns of (d) IL-NTA, (f) IL-EDTA, and (h) IL-DTPA, revealing the existence of a number of constituents in nanocrystalline form.

Fig. 4. Oxidation states of inorganic layer formed by PE: (a) High-resolution XPS spectra of three different inorganic layers for Al 2p. Deconvolution analysis on Al 2p spectra of (b) ILNTA, (c) IL-EDTA, and (d) IL-DTPA. Al2O3/Al refers to anodic Al2O3 commonly found at oxide-metal interface. (e) High-resolution XPS spectra of three different inorganic layers for Si 2p. Deconvolution analysis on Si 2p spectra of (f) IL-NTA, (g) IL-EDTA, and (h) ILDTPA. SiOx refers to compounds having Si-O bonds whose the elemental ratio is typically unresolved.

Fig. 5. Anti-corrosion and catalytic performances of inorganic layer formed by PE: (a) Potentiodynamic polarization curves of IL-NTA, IL-EDTA, and IL-DTPA after PE treatment for 300 s taken after 1 and 24 h of immersion in 3.5 wt.% NaCl solution with a direct comparison with the corrosion responses of bare Al 6061 alloy. (b) Impedance responses of all inorganic layers in 3.5 wt.% NaCl solution presented in the Nyquist plot. The construction of equivalent circuit models considers the microstructural complexity of inorganic layers. UV-vis absorbance profiles of MB solution in the presence of H2O2 treated with (c) IL-NTA, (d) IL-EDTA, and (e) IL-DTPA. Inset: photographic observation of MB solution after catalysis from 0 to 48 h. (f) Catalytic efficiencies of IL-NTA, IL-EDTA, and IL-DTPA, which are compared with that of inorganic layer made without complexing agents (IL). Inset: comparison of MB solutions after 48 h incubation with: IL, IL-NTA, IL-EDTA, and ILDTPA. (g) Reusability test results of inorganic layers over five cycles of immersion in MB solution for 48 h. (h) Catalytic mechanism of inorganic layer for the degradation of MB.

Fig. 6. Electrochemical roles of complexing agent on formation mechanism of inorganic layer by PE. (a) Density distributions of frontier molecular orbitals in NTA, EDTA, and DTPA after full deprotonation of -COOH groups in the present PE electrolyte having a strong alkalinity. Grey: carbon, white: hydrogen, red: oxygen, blue: nitrogen. (b) Energy levels of HOMO and LUMO of different complex ions to highlight the tendency for establishing donor-acceptor interaction with inorganic surface. (c) Electrochemical modeling of inorganic layer during PE based on impedance measurements by considering structural inhomogeneity (inset). Nyquist plots show three R-C responses from EDL, inorganic layer, and electrolytemetal interface. Interaction of complexing agents and inorganic surface, which include donation of lone electron pairs, are shown. (d) Schematic illustration showing the reconstruction of EDL in electrolytes containing three different complexing agents and their effects on the generation of soft plasma discharges underlying the formation of inorganic layers.

Tables Table 1 Relative fractions of Al-containing compounds in inorganic layers based on deconvolution of XPS peaks from Al 2p. Compound

IL-NTA

IL-EDTA

IL-DTPA

γ-Al2O3

0.74

0.65

0.31

Al(OH)3

0.13

-

-

Al2Si2O13

-

0.30

0.62

Al2O3/Al

-

0.05

0.07

SiO2(Al2O3)2.1

0.13

-

-

Table 2 Relative fractions of Si-containing compounds in inorganic layers based on deconvolution of XPS peaks from Si 2p. Compound

IL-NTA

IL-EDTA

IL-DTPA

SiO2

0.46

0.47

0.36

SiOx

0.34

0.17

0.17

Al2Si2O13

-

0.36

0.47

SiO2(Al2O3)2.1

0.20

-

-

Table 3 Potentiodynamic polarization results of Al 6061 alloy substrate protected by different inorganic layers measured from -0.2 to 0.4 V vs open circuit potential in 3.5 wt.% NaCl solution. Sample Al 6061 alloy

Immersion time (h)

Ecorr (mV)

Icorr (A cm-2)

01

-440

5.77 × 10

24

-1089

4.45 × 10

01

-419

6.87 × 10

24

-846

4.36 × 10

01

-366

7.69 × 10

24

-698

3.15 × 10

01

-304

2.59 × 10

24

-599

9.37 × 10

-8 -6 -9

IL-NTA

-7 -9

IL-EDTA

-8

-10

IL-DTPA

-10

βa (V decade-1) -1

7.73 × 10

-1

6.71 × 10

-1

4.86 × 10

-1

6.43 × 10

-1

7.73 × 10

-1

1.94 × 10

-1

7.21 × 10

-1

4.45 × 10

βc (V decade-1) -1

-3.68 × 10

-1

-2.75 × 10

-1

-1.70 × 10

-1

-5.84 × 10

-1

-3.68 × 10

-1

-8.94 × 10

-1

-2.22 × 10

-1

-5.10 × 10

Rp (Ω cm2) 6

1.88 × 10

4

1.90 × 10

6

7.69 × 10

5

3.03 × 10

7

3.98 × 10

6

1.92 × 10

8

5.39 × 10

8

1.63 × 10

Table 4 Electrochemical impedance parameters of inorganic layers measured from 106 to 0.1 Hz in 3.5 wt.% NaCl solution. Sample

Rs (Ω cm2)

IL-NTA

17.50

IL-EDTA IL-DTPA

Rsurf (Ω cm2)

nsurf

Ysurf (S sn cm-2)

Rout (Ω cm2)

1.31  10

0.58

2.42 10

-6

8.52  10

17.50

32.40

0.57

2.54  10

-6

1.91  10

17.50

-

-

-

7.41  10

2

nil

Yil (S sn cm-2)

Rint (Ω cm2)

nint

Yint (S sn cm-2)

2

0.82

7.45  10

-6

1.24  10

4

0.57

3.01  10

5

0.82

6.29  10

-7

5.61  10

5

0.47

4.93  10

5

0.94

2.25  10

-8

4.96  10

6

0.79

7.48  10

-5

-7

-8

Table 5 Quantum chemical parameters drawn from the DFT simulation of different complex ions after deprotonation in strong alkaline condition. γ

Ions

EHOMO (eV)

ELUMO (eV)

∆E (eV)

µ (D)

A

I

X (eV)

(eV)

NTA-3

-5.81

0.34

6.15

5.05

0.34

5.81

3.07

2.73

1.4 × 10-2

EDTA-4

-5.44

0.28

5.72

5.82

0.28

5.44

2.86

2.58

3.4 × 10-2

DTPA-5

-5.06

0.25

5.31

6.89

0.25

5.06

2.65

2.40

5.5 × 10-2

∆N

Table 6 Electrochemical impedance parameters associated with the surface change of inorganic layers in PE electrolyte with different complexing agents. τdl (s)

Cdl (F)

Electrolyte

Rs (Ω cm2)

Rct (Ω cm2)

ndl

Ydl (S sn cm-2)

NTA

50

4.72 × 104

0.98

6.01 × 10-10

3.65 × 10

EDTA

50

1.34 × 105

0.98

1.08 × 10-11

5.08 × 10

DTPA

50

4.34 × 105

0.99

2.76 × 10-12

2.76 × 10

-10 -11 -12

-5

1.72 × 10

-6

6.80 × 10

-6

1.20 × 10

Declaration of interests

☒

The authors declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Research Highlights  Destructive nature of high-energy plasma in water is controlled by modulating EDL  Porosity of Al2O3-based inorganic layer is adjustable by soft plasma discharges  Corrosion is delayed by aluminosilicate compounds with compact structure  Catalytic activity towards organic dye is enabled via homolytic cleavage of H2O2  An optimum combination of chemical stability and catalytic activity is achieved