Effect of Negative Current on the Microstructure of Oxide Coatings Prepared by Hybrid Pulse Anodization

metals

Article Effect of Negative Current on the Microstructure of Oxide Coatings Prepared by Hybrid Pulse Anodization

Shuo Huang 1, Bailing Jiang 1,2,*, Cancan Liu 1,*, Qingying Shao 1 and Hongtao Li 1 1 College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211816, China; [email protected] (S.H.); [email protected] (Q.S.); [email protected] (H.L.) 2 College of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China * Correspondence: [email protected] (B.J.); [email protected] (C.L.); Tel.: +86-136-6924-7719 (B.J.); +86-188-5160-4703 (C.L.) Received: 27 November 2018; Accepted: 22 December 2018; Published: 27 December 2018

Abstract: The oxide coatings were prepared on 6061 Al alloy at different negative current densities in oxalic acid using the hybrid pulse anodization (HPA) method at room temperature. The variation curves of positive and negative voltages with anodization time were recorded. The nanopore diameters and distribution regularities in HPA coatings were analyzed with the Image-Pro Plus software based on ﬁeld-emission scanning electron microscope (FE-SEM) images. The results showed that the negative current could reduce the growth rate of HPA coatings, and thus led to a small thickness of the coatings within the same anodization time. Besides, appropriate negative current densities resulted in the better distribution uniformity of nanopores, but the excessive negative current densities tended to cause inferior nanopore arrangement. These were attributed to the existence of the negative current, causing H+ and O2− to move in opposite directions, so that a large number of H+ concentrated on the surface of the HPA coatings, resulting in the accelerated dissolution of the coatings.

Keywords: aluminum alloy; hybrid pulse anodization; microstructure; negative current

1. Introduction In the past decades, porous anodic alumina (PAA) has attracted great attention as templates to produce different kinds of functional nanostructures such as nanowires and nanotubes [1]. It has been in widespread use in biological sensors [2], membrane reactors [3], energy storage devices [4], and super capacitors [5] for the regular and controllable nanopores. Most PAA coatings were fabricated using direct-current anodization (DCA) at potentiostatic mode in sulfuric acid (H2SO4)[6,7], oxalic acid (H2C2O4)[8–10], phosphoric acid (H3PO4)[11], chromic acid (H2CrO4)[12], and mixed acid electrolytes [13,14]. Much work have been done to explore the mechanism [15,16] of anodization. Besides, the inﬂuence of anodization parameters (such as potential [17,18], current density [19], electrolyte temperature, and anodization time [20]) on the microstructure and properties of PAA coatings has been also intensively studied. Studies found that during the anodization process, the growth and dissolution of aluminum oxide happened simultaneously and competed with each other. The Joule heat generated in anodization could accelerate the dissolution and destroy the uniformity of the PAA coatings [21]. Hence, DCA was generally carried out at low temperature (0–10 ◦C) to suppress the Joule heat. Cooling the electrolytes from room temperature was a time-consuming and cost-increasing process. Moreover, the low temperature was inclined to reduce the growth rate of PAA coatings, and in some cases, the processing time was even prolonged to dozens of hours [8].

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In order to overcome these limitations in conventional DCA, pulse anodization (PA) was developed for the advantage of timely heat dissipation [21]. Many studies have investigated the effects of PA on the microstructure and properties of PAA coatings [22]. Bozza et al. [23] has reported that fewer defects were found at the oxide/metal interface proceeding with PA compared to DCA. Additionally, negative voltage was introduced into the PA process to improve the regularity of nanopores by suppressing the anodic current during the pulse-free time [24]. Chung et al. [25] has demonstrated that hybrid pulse anodization (HPA), which is pulse voltage including positive voltage together with low negative voltage (2V), could suppress the generation of Joule heat to reduce the dissolution rate of HPA coatings, and result in the better regularity of nanopores at room temperature [26]. In addition, Chung et al. [27] has demonstrated that a short pulse-off stage and low current density resulted in a better distribution and uniformity of nanopores in oxide coatings; on the country, the effective combination of a moderate duty cycle and high current density was able to accelerate the growth of HPA coatings. Although many works have proved that the HPA could enhance the distribution uniformity of the nanopores and improve the growth rate of oxide coatings, the role of negative current in HPA and the inﬂuence mechanisms of negative current are still not fully understood. In this paper, the oxidation coatings were prepared at different negative currents using the HPA method. The effects of negative current on the microstructures and growth rates of oxidation coatings were analyzed, and the inﬂuence mechanisms were proposed.

2. Materials and Methods

2.1. HPA Process First, 6061 alloy (0.8–1.2% Mg, 0.4–0.8% Si, 0.7% Fe, 0.15–0.4% Cu, 0.04–0.35% Cr, 0.25% Zn, 0.15% Mn, 0.15% Ti, and balance Al) plates with a thickness of 3 millimeter and diameter of 35 mm were employed as substrate materials, and the area of the aluminum sample for HPA was 20 cm2. Prior to the anodic oxidation process, specimens were ﬁrstly ground with SiC abrasive paper to 3000 mesh, and then immersed in 0.5 M of NaOH for 1 min at 50 ◦C to remove the natural oxide ﬁlm. Subsequently, the specimens were rinsed with de-ionized water and dried in the air. The HPA process was carried out for 30 min in 0.3 M of oxalic acid electrolyte (C2H2O4·2H2O) with a volume of 4 liters in a rectangular plastic container using a bipolar pulse power supply. Figure1 showed the schematic of the experimental set-up. Aluminum was set as the anode, and the stainless steel plate larger than the aluminum sample was set as the cathode. The distance between them was 5 centimeters. The temperature of the electrolyte was kept at 15 ± 3 ◦C by an air-blowing device to stir the electrolyte and an external circle cooling system simultaneously. The ton/toff, positive/negative pulse width 2 ratio (t+/t−) and positive current density were set as 0.18, 1, and 5 A/dm , respectively. The negative current densities varied from 0 to 3 A/dm2 with an increment of 0.5 A/dm2. After the HPA treatment, samples were ultrasonic cleaned in ethanol for 5 min and dried in the air.

2.2. Characterization The thickness of the oxide coatings was measured with a handheld thickness gauge (FMP20, Helmut Fischer GMBH, Sindelﬁngen, Germany). The positive and negative voltages were recorded manually over the anodization time. The surface and cross-sectional morphologies of the anodized specimens were observed under a ﬁeld-emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Tokyo, Japan). The diameters and distribution of the nanopores in oxide coatings were analyzed with the software of Image-Pro Plus (Media Cybernetics, Rockville, MD, USA) based on FE-SEM images. Metals 2019, 9, 22 3 of 9 Metals 2019, 9, x FOR PEER REVIEW 3 of 9

Figure 1. The schematic of the experimental set-up.

3. Results 3.1. Effect of Negative Current on Anodization Voltage 3.1. Effect of Negative Current on Anodization Voltage The variation curves of the positive and negative voltages with anodization time are presented in The variation curves of the positive and negative voltages with anodization time are presented Figure2. It can be seen that regardless of the negative current densities, the positive voltages exhibited in Figure 2. It can be seen that regardless of the negative current densities, the positive voltages similar variation trends. In the initial stage, the positive voltages rose rapidly to the peak, and then exhibited similar variation trends. In the initial stage, the positive voltages rose rapidly to the peak, went down sharply. Subsequently, they kept increasing gradually until the end of HPA. Similar and then went down sharply. Subsequently, they kept increasing gradually until the end of HPA. variation trends were observed for the negative voltages in Figure2c. In addition, the peak values were Similar variation trends were observed for the negative voltages in Figure 2c. In addition, the peak obtained between 10 and 20 s (Figure2b) due to the formation of a continuous and dense imperforate values were obtained between 10 and 20 seconds (Figure 2b) due to the formation of a continuous barrier layer, which had much higher resistance than the aluminum substrate [21,28]. The time that it and dense imperforate barrier layer, which had much higher resistance than the aluminum substrate took to reach the peak voltage was prolonged as the negative current density increased. Furthermore, [21,28]. The time that it took to reach the peak voltage was prolonged as the negative current density the highest peak voltage of 140 V was found on the curve without the negative current, and the others increased. Furthermore, the highest peak voltage of 140 V was found on the curve without the declined as the negative current densities rose from 0 to 3 A/dm2. This was because the thickness of negative current, and the others declined as the negative current densities rose from 0 to 3 A/dm2. the barrier layer was reduced by the increased negative current density. After the voltages reached the This was because the thickness of the barrier layer was reduced by the increased negative current peak, the nanopores started to appear due to the heterogeneous dissolution in the barrier layer [29]. density. After the voltages reached the peak, the nanopores started to appear due to the At some points with small impurities and defects on the coating surface, the dissolution effect was heterogeneous dissolution in the barrier layer [29]. At some points with small impurities and defects enhanced to be stronger than the other areas of the surface. The current was concentrated in these on the coating surface, the dissolution effect was enhanced to be stronger than the other areas of the places and the dissolution rate was accelerated; thus, the nanopores were formed [30]. As the coating surface. The current was concentrated in these places and the dissolution rate was accelerated; thus, thickness at the nanopores decreased, the resistance of the coatings began to reduce, and therefore, the nanopores were formed [30]. As the coating thickness at the nanopores decreased, the resistance the voltage variation curves showed a downward tendency. As the HPA progressed, the nanopores of the coatings began to reduce, and therefore, the voltage variation curves showed a downward continued to grow competitively until they achieved regular arrangement [15]. The voltages rose tendency. As the HPA progressed, the nanopores continued to grow competitively until they slowly along with the increase of the coating thickness until the end of anodization. Obviously, the ﬁnal achieved regular arrangement [15]. The voltages rose slowly along with the increase of the coating voltages were different (Figure2a,c), which were closely related to the coating thickness. thickness until the end of anodization. Obviously, the final voltages were different (Figure 2a,c), which were closely related to the coating thickness.

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FigureFigure 2. 2.VariationVariation curves curves ofof positive positive (a (,ba,)b and) and negative negative (c, (dc,)d voltages) voltages with with anodization anodization time time during during hybridhybrid pulse pulse anodization anodization (HPA) (HPA) process process for for 30 30 minutes. min. 3.2. Effect of Negative Current on Microstructures of HPA Coatings 3.2. Effect of Negative Current on Microstructures of HPA Coatings Figure3 displays the thickness of HPA coatings that were prepared at different negative current Figure 3 displays the thickness of HPA coatings that were prepared at different negative current densities for 30 min. It could be found that the coating thickness decreased gradually with the densities for 30 minutes. It could be found that the coating thickness decreased gradually with the increasing negative current densities. It was about 40 µm without negative current, while it reduced increasing negative current densities. It was about 40 µm without negative current, while it reduced to 23 µm at the negative current density of 3.0 A/dm2. The variation trends of coating thickness to 23 µm at the negative current density of 3.0 A/dm2. The variation trends of coating thickness were were consistent with the values of the final voltages in Figure2. Due to that, the coating growth consistent with the values of the final voltages in Figure 2. Due to that, the coating growth rate was rate was determined by an intense competition between the formation and dissolution of coatings, determined by an intense competition between the formation and dissolution of coatings, which which proceeded simultaneously. The two inverse processes could be expressed in formulas (1) proceeded simultaneously. The two inverse processes could be expressed in formulas (1) and (2), and (2), respectively [31]. The higher negative current density caused more H+ to move toward respectively [31]. The higher negative current density caused more H+ to move toward the the coating/electrolyte interface under the action of an external electric field, thus accelerating the coating/electrolyte interface under the action of an external electric field, thus accelerating the dissolution of the HPA coatings and ultimately leading to a decrease in the growth rate of the coatings. dissolution of the HPA coatings and ultimately leading to a decrease in the growth rate of the coatings. 2Al + 3[O] → Al2O3 + 1424 J/mol (1) 2Al + 3[O] → Al2O3 + 1424 J/mol (1) + 3+ Al2O3 + 6H → 2Al + 3H2O (2) Al2O3 + 6H+ → 2Al3+ + 3H2O (2) The surface SEM micrographs of anodization coatings prepared at different negative current densitiesThe surface for 30 SEM min aremicrographs presented of in anodization Figure4. As cancoat beings seen, prepared when thereat different was no negative negative current current densitiesduring thefor HPA30 minutes process, are the presented surface ofin the Figure anodization 4. As can coating be seen, was when smooth, there and was showed no negative flowing currentmorphology. during Inthe addition, HPA process, the coating the surface was compact of the withanodization small-sized coating nanopores was smooth, that were and irregular showed in flowingshape. morphology. When 1 A/dm In2 addition,of negative the current coating density was compact was applied, with thesmall-sized nanopores nanopores got obviously that were much irregularlarger than in shape. those inWhen Figure 1 4A/dma,b. Continuously2 of negative increasingcurrent density the negative was applied, current densitythe nanopores to 2 A/dm got 2, obviouslythe nanopores much turnedlarger athan little those bit larger. in Figure Besides, 4a,b. the Continuously surface of the increasing coating became the negative much flatter current with densityless sediment, to 2 A/dm and2, the the nanopores nanopores turned changed a little to have bit larger. better distributionBesides, the uniformity.surface of the It indicatedcoating became that the muchapplication flatter with of negative less sediment, current and expanded the nanopore the nanoporess changed and to obtained have better a well-ordered distribution configuration.uniformity. It indicated that the application of negative current expanded the nanopores and obtained a well-ordered configuration. However, a large number of granular and interlacing stripe convexities

Metals 2019, 9, 22 5 of 9 Metals 2019, 9, x FOR PEER REVIEW 5 of 9 Metals 2019, 9, x FOR PEER REVIEW 5 of 9 However,appeared on a large the surface number of ofanodization granular and coatings interlacing when the stripe negative convexities current appeared density reached on the surface 3 A/dm of2. 2 anodizationItappeared caused onthe coatings the coating surface when surface of theanodization negativeto be extremely currentcoatings density unwheneven reachedthe (Figure negative 3 A/dm4g,h). current Beyond. It causeddensity that, the reached coatingthe nanopores 3 surfaceA/dm2. toarrangementIt becaused extremely the turned coating uneven disordered, surface (Figure to4 andg,h). be theextremely Beyond nanopore that, un evendiameter the nanopores(Figure decreased 4g,h). arrangement markedly.Beyond that, turned the disordered, nanopores andarrangement the nanopore turned diameter disordered, decreased and the markedly. nanopore diameter decreased markedly.

Figure 3. The thickness of HPA coatings prepared at different negative curre nt densities for 30 minutes.FigureFigure 3. The thicknessthickness ofof HPA HPA coatings coatings prepared prepared at differentat different negative negative current curre densitiesnt densities for 30 for min. 30 minutes.

Figure 4.4. Surface SEM micrographs of anodization coatingscoatings prepared at different negative current densitiesFigure 4. forSurface 30 min:minutes: SEM 0 A/dm micrographs 0 A/dm2 (a,2b (),a,b 1 of), A/dm 1anodization A/dm2 (c2 ,d(c),,d 2), coatin A/dm2 A/dmgs2 (2preparede (,ef,)f and) and 3 3at A/dm A/dm different22 ((gg,,hh negative).). current densities for 30 minutes: 0 A/dm2 (a,b), 1 A/dm2 (c,d), 2 A/dm2 (e,f) and 3 A/dm2 (g,h). In orderorder to to further further understand understand the the nanopore nanopore diameters diameters and theirand distribution,their distribution, the FE-SEM the FE-SEM images inimages FigureIn orderin4a,c,e,g Figure to were further4a,c,e,g analyzed understand were with analyzed the the Image-Pro nanoporewith the Plus Image-Prodiameters software Plusand to identify softwaretheir distribution, their to ownidentify characteristics. the their FE-SEM own Ascharacteristics.images shown in inFigure Figure As 4a,c,e,gshown5, without inwere Figure negative analyzed 5, without current, with thenega the range tiveImage-Pro current, of 20 ± Plus the5 µ m rangesoftware occupied of 20 to the± identify5 mainµm occupied distribution their own the ofmaincharacteristics. all distribution the nanopores. As ofshown all With the in nanopores. theFigure increase 5, without With of negative thenega increasetive current current, of negative densities the range current to of 1 20 A/dm densities± 5 µm2 and occupied to 2 1 A/dm A/dm the22, theandmain diameters 2 distributionA/dm2, the of nanopores diameters of all the were ofnanopores. nanopores mainly distributedWith were the mainly increase in 30distµ ributedmof andnegative 40in µ30m, currentµm respectively. and densities 40 µm, It respectively. implied to 1 A/dm that2 theItand implied greater 2 A/dm that the2, the negativethe diameters greater current the of negativenanopores density, current the were larger mainlydensity, the nanoporedist theributed larger diameter. thein 30 nanopore µm However, and diameter.40 µm, as respectively. the However, negative currentasIt impliedthe negative density that the increasedcurrent greater de to nsitythe 3A/dm negative increased2, the current nanopore to 3A/dm density, diameter2, the the nanopore larger became the muchdiameter nanopore smaller became diameter. instead much (Figure However, smaller5d). insteadas the negative (Figure 5d).current density increased to 3A/dm2, the nanopore diameter became much smaller instead (Figure 5d).

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Figure 5. Pore diameter distribution diagramsdiagrams of anodization coatings prepared at different negative current densities for 30 min:minutes: 0 A/dm 0 A/dm2 (a),2 ( 1a), A/dm 1 A/dm2 (b2 ),(b 2), A/dm 2 A/dm2 (2c ()c and) and 3 3 A/dm A/dm22( (dd).).

4. Discussion Figure6 6exhibits exhibits the the schematic schema diagramstic diagrams of two of different two different current modescurrent (PA modes and HPA).(PA and Compared HPA). withCompared traditional with DCA, traditional the intermittent DCA, the workingintermittent principle working was utilizedprinciple in was the PAutilized process in (Figurethe PA6 processa). In a cycle, there was enough pulse-off time (t ) to dissipate the Joule heat generated (Formula (1)) in the (Figure 6a). In a cycle, there was enoughoff pulse-off time (toff) to dissipate the Joule heat generated anodization process (t ). Figure6b shows the anodization current mode of HPA with a negative current. (Formula (1)) in the anodization+ process (t+). Figure 6b shows the anodization current mode of HPA Thewith negativea negative current current. was The applied negative tosuppress current was the applied Joule heat to suppre generationss the by Joule eliminating heat generation the anodic by current during t [30,31]. On the one hand, the dissolution reaction (Formula (2)) was weakened eliminating the anodicoff current during toff [30,31]. On the one hand, the dissolution reaction (Formula due(2)) was to the weakened timely dissipation due to the of timely Joule heat;dissipation this was of beneficial Joule heat; to decreasethis was thebeneficial dissolution to decrease rate of the coatingsdissolution and rate lead of to the an coatings increasing and growth lead rateto an of increasing anodizing growth coatings rate [22 ,of24 ].anodizing On the other coatings hand, [22,24]. due to 2− + theOn the conversion other hand, of positive due to the and conversion negative poles of po ofsitive power and supply, negative O polesand of H powermigrated supply, to the O2− cathode and H+ andmigrated anode, to respectively. the cathode Itand resulted anode, in respectively. the enhancement It resulted of the in dissolution the enhancement effect. There of the was dissolution a balance betweeneffect. There the twowas sidesa balance to get between a well-ordered the two nanoporesides to get configuration. a well-ordered nanopore configuration. In a pulse period, H+ and O2− moved toward the cathode (steel plate) and the anode (Al substrate) separately when the positive current was applied at the beginning (Figure7a). Ultimately, the O 2− gathered around the anode (Figure7c), and they were in direct contact with the Al substrate; then, Formula (1) occurred with Joule heat production. After that, the positive and negative poles of power supply switched, and H+ and O2− moved in opposite directions under the action of negative electric field (Figure7b). The anodization process in Formula (1) stopped, and the Joule heat generated was + taken away by water cooling during t− and toff. Large numbers of the H gathered on the surface of the coatings and in the nanopores (Figure7d), accelerating the dissolution of the HPA coatings and playing the role of pore expansion and leveling the surface of coatings (Formula (2)) [28]. Besides, the greater the negative current was, the larger the driving force for H+. More H+ accumulated on the surface of the oxide coatings at the same time, making the dissolution rate of the coatings faster. On this

Figure 6. Schematic diagram of two different current modes: (a) pulse anodization (PA) and (b) HPA.

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Figure 5. Pore diameter distribution diagrams of anodization coatings prepared at different negative current densities for 30 minutes: 0 A/dm2 (a), 1 A/dm2 (b), 2 A/dm2 (c) and 3 A/dm2 (d).

4. Discussion Figure 6 exhibits the schematic diagrams of two different current modes (PA and HPA). Compared with traditional DCA, the intermittent working principle was utilized in the PA process (Figure 6a). In a cycle, there was enough pulse-off time (toff) to dissipate the Joule heat generated (Formula (1)) in the anodization process (t+). Figure 6b shows the anodization current mode of HPA with a negative current. The negative current was applied to suppress the Joule heat generation by eliminatingMetals 2019, 9, 22the anodic current during toff [30,31]. On the one hand, the dissolution reaction (Formula7 of 9 (2))Metals was 2019 weakened, 9, x FOR PEER due REVIEW to the timely dissipation of Joule heat; this was beneficial to decrease7 ofthe 9 dissolution rate of the coatings and lead to an increasing growth rate of anodizing coatings [22,24]. Onoccasion, theIn othera thepulse hand, nanopore period, due diametersto H the+ and conversion O became2− moved of larger, po towardsitive and and athe better negative cathode nanopore poles (steel of distribution plate)power and supply, uniformitythe Oanode2− and was (AlH+ + migratedobtained.substrate) to However,separately the cathode excessive when and the anode, H positivecaused respectively. current serious was damage It appliedresulted to theat in the nanoporethe beginning enhancement arrangement. (Figure of 7a).the Beyond dissolutionUltimately, that, effect.the thicknessO2 −There gathered was obviously arounda balance tended the between anode to get (Figure the smaller two 7c), sides because and to they get of awere the well-ordered enhancement in direct contactnanopore of the with coating configuration. the Al dissolution. substrate; then, Formula (1) occurred with Joule heat production. After that, the positive and negative poles of power supply switched, and H+ and O2− moved in opposite directions under the action of negative electric field (Figure 7b). The anodization process in Formula (1) stopped, and the Joule heat generated was taken away by water cooling during t− and toff. Large numbers of the H+ gathered on the surface of the coatings and in the nanopores (Figure 7d), accelerating the dissolution of the HPA coatings and playing the role of pore expansion and leveling the surface of coatings (Formula (2)) [28]. Besides, the greater the negative current was, the larger the driving force for H+. More H+ accumulated on the surface of the oxide coatings at the same time, making the dissolution rate of the coatings faster. On this occasion, the nanopore diameters became larger, and a better nanopore distribution uniformity was obtained. However, excessive H+ caused serious damage to the nanopore arrangement. Beyond that, the thickness obviously tended to get smaller because of the enhancement of the coating dissolution. Figure 6.6. SchematicSchematic diagramdiagram of of two two different different current current modes: modes: (a) pulse(a) pulse anodization anodization (PA) (PA) and (band) HPA. (b) HPA.

Figure 7.7. Schematic diagram of ion movements and distributionsdistributions during the anodization process without (a,,cc)) andand withwith ((b,,dd)) negativenegative current.current.

5. Conclusions 5. Conclusions Anodization coatings have been successfully fabricated fabricated using using the the HPA HPA method in in oxalic acid. acid. Results showed that the negative current could re reduceduce the growth rate of anodization coatings, leading toto the the small small thickness thickness coatings coatings within within the same the treatingsame treating time. The time. coating The thickness coating decreasedthickness µ 2 fromdecreased 40 to 23fromm 40 as to the 23 negative µm as the current negative densities current increased densities from increased 0 to 3 A/dm from .0 Besides, to 3 A/dm the2 nanopore. Besides, µ 2 diametersthe nanopore increased diameters from increased 20 to 40 mfrom with 20 the toincrease 40 µm with of negative the increase current of densities negative from current 0 to 2densities A/dm . Appropriatefrom 0 to 2 A/dm negative2. Appropriate current densities negative played current the roledensities of reaming played pores the androle levelingof reaming surfaces pores to and get betterleveling nanopore surfaces distributionto get better nanopore uniformity. distribution However, uniformity. the surface However, ﬂatness ofthe the surface oxide flatness coating of was the damagedoxide coating seriously was damaged at excessive seriously negative at excessive current densities, negative andcurrent inferior densities, nanopore and arrangementinferior nanopore was obtainedarrangement during was the obtained HPA process. during the HPA process.

Author Contributions: TheThe work work was was done done in in cooperation cooperation with with the the join jointt efforts efforts of all of the all theauthors. authors. B.J., B.J.,H.L. H.L.and andC.L. C.L.designed designed the theexperiments; experiments; S.H. S.H. and and Q.S. Q.S. performed performed the the experiments; experiments; S.H., S.H., Q.S. Q.S. and and C.L. C.L. analyzed the experimental data; S.H. wrote the paper. experimental data; S.H. wrote the paper.

Funding: This research was funded by the National Natural Science Foundation of China (No. 51571114).

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Funding: This research was funded by the National Natural Science Foundation of China (No. 51571114). Acknowledgments: The authors gratefully acknowledge Dichun Chen for providing the FE-SEM. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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