Oxide Film Efficiently Suppresses Dendrite Growth in Aluminum-IonBatteryHao Chen, Hanyan Xu, Bingna Zheng, Siyao Wang, Tieqi Huang, Fan Guo, Weiwei Gao,and Chao Gao*

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, KeyLaboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road,Hangzhou 310027, P. R. China

*S Supporting Information

ABSTRACT: Aluminum metal foil is the optimal choice as an anode material foraluminum-ion batteries for its key advantages such as high theoretical capacity, safety, andlow cost. However, the metallic nature of aluminum foil is very likely to induce severedendrite growth with further electrode disintegration and cell failure, which is inconsistentwith previous reports. Here, we discover that it is aluminum oxide film that efficientlyrestricts the growth of crystalline Al dendrite and thus improves the cycling stability of Alanode. The key role of surficial aluminum oxide film in protecting Al metal anode lies indecreasing the nucleation sites, controlling the metallic dendrite growth, and preventing theelectrode disintegration. The defect sites in aluminum oxide film provide channels forelectrolyte infiltration and further stripping/depositing. Attributed to such a protectivealuminum oxide film, the Al−graphene full cells can attain up to 45 000 stable cycles.

KEYWORDS: aluminum dendrite, aluminum oxide film, aluminum metal anode, aluminum-ion battery,aluminum anode reaction mechanism

■ INTRODUCTIONRechargeable aluminum-ion battery (AIB) is a promisingcandidate for future energy storage technologies due to itsimpressive advantages such as high anode capacity, cost-effectiveness, and safety.1−3 Its key advantages mostly benefitfrom unique properties of the ideal aluminum metal anode:low-cost, abundance, incombustibility, air stability, highgravimetric capacity of 2980 mAh g−1, and volumetric capacityof 8040 mAh cm−3. However, progress regarding AIBs overpast 30 years has been impeded by critical problems such aslack of suitable electrolyte or cathode materials.4 A recentbreakthrough reported by Dai et al.5 opens a new avenue forthis stagnant field by employing an AlCl3/[EMIm]Cl ionicliquid electrolyte and graphitic cathode. This pioneering workstimulates various research enthusiasms in cathode materials ofAIB such as graphite,6−8 graphene,9−12 carbon,13−15 and sulfur-based materials.16−18

In contrast to the significant progresses regarding cathodematerials3 and ionic liquid electrolyte,19,20 an in-depth study onaluminum anodes of AIBs has not been done. Importantly, themetallic nature of aluminum is very likely to induce dendriteformation during reversible aluminum plating/stripping,21,22

which is analogous to the well-known problem of lithiumdendrite hindering the practical application of ultimate lithium-metal anode of Li-ion batteries.23 Those fatal dendrites maypierce the separator, causing safety hazards and leading toanode disintegration and further cell failure. However, mostrecent reports on AIBs exhibited an absence of dendrites during

cycling and even dendrite-free behavior of cycled aluminummetal anode, which is inconsistent with the metallic nature ofaluminum foil anodes and various reports on electrodepositionof aluminum species.21,22,24 Accordingly, it is of significance tostudy such a dendrite puzzle associated with aluminum metalanodes.Here, we show detailed insight into the behavior of

aluminum metal anodes of AIBs during cycling. Our studyreports the first observation of an ultrathin yet uniformprotective aluminum oxide layer covering the aluminum metalanode to effectively suppress dendrite growth and preventelectrode disintegration. Constituents of this first-discoveredprotective layer and detailed reaction pathway through thisprotective layer are also revealed. Benefiting from thisprotective layer, the aluminum metal anode can be reversiblyand stably cycled to support the long cycle life of Al−graphenefull battery up to 45 000 cycles, far surpassing that without thisprotective layer.

■ EXPERIMENTAL METHODSMaterials Preparation. Normal aluminum foil (Al-n, purchased

from MTI, thickness of 20 μm) was used as received. The aluminumanode control sample, whose surficial oxidation film was removedbeforehand, was named Al-r. The oxidation film was removed by a

Received: May 18, 2017Accepted: June 21, 2017Published: June 21, 2017

Research Article

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© 2017 American Chemical Society 22628 DOI: 10.1021/acsami.7b07024ACS Appl. Mater. Interfaces 2017, 9, 22628−22634

widely used method: Al foil was immersed in mixture of 6% H3PO4and 1.8% H2CrO4 solution at 60 °C for 30 min,25 fast washed withargon-saturated anhydrous ethanol, and then transferred into theglovebox. A graphene aerogel cathode with few defects was preparedby freeze-drying of graphene oxide solution26 followed by thermalannealing at 2000 °C.11 The electrolyte was made by mixing 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl, 97%, Acros Chemicals,previously heated in vacuum at 130 °C for 24 h) and 1.3 mol equivanhydrous aluminum chloride (AlCl3, 99.999%, Sigma-Aldrich, used asreceived) in a glovebox for 12 h to obtain transparent yellow liquid.The cycled aluminum foils from dissembled cells were washed withargon-saturated anhydrous dichloromethane27 in the glovebox toremove residual electrolyte and then vacuumed in the glovebox toremove dichloromethane.Electrochemistry. The cells were assembled in glovebox in both a

coin-type cell and a soft pack-type cell. The aluminum symmetric cellwas fabricated into soft pack cell by using either Al-n or Al-r as bothworking electrode and counter electrode with glassy fiber as separatorand AlCl3/[EMIm]Cl ionic liquid as electrolyte. Cycling stability andcycle life of the anodes were evaluated by employing a symmetric cell,and the cells were then charged and discharged at 1 mA cm−2 for 0.5 hor 5 mA cm−2 for 0.25 h in each half cycle until cell failure. Cyclicvoltammetry (CV) measurements were performed in two-electrodemode at different scan rates. The Al−graphene full cell was preparedby using graphene aerogel with few defects as cathode, Al-n or Al-r foilas anode with glassy fiber as separator, and AlCl3/[EMIm]Cl ionicliquid as electrolyte. The long-term galvanostatic discharge/charge ofthe full cell was tested by cycling under a voltage range of 0.7−2.51 V.Characterization. The morphologies of the samples were

investigated through a scanning electron microscope (SEM, HitachiS-3000N). The elemental mapping results were examined through an

energy-dispersive spectrometer (EDS) attached to Hitachi S-3000N.Powder X-ray diffraction (XRD) data were collected on a Bruker D8X-ray diffractometer with Cu Kα1 radiation (1.5405 Å) in the scanrange of 10−80°. X-ray photoelectron spectroscopy (XPS) analysisresults were obtained on an Escalab250Xi spectrometer. The dischargeand charge measurements at room temperature were carried out on aLand BT2000 Battery Test System (Wuhan, China). CV wasperformed on a CHI600D Electrochemical Workshop (Shanghai,China) at a scan rate of 1−10 mV s−1.

■ RESULTS AND DISCUSSIONFigure 1a exhibits the SEM image of Al-n foil anode obtainedfrom the dissembled Al−graphene cell11 after 1000 cycles,showing a plain surface without obvious corruption sites oraluminum dendrites observed. This phenomenon is consistentwith many recently published reports on AIBs, that an apparentdendrite-free flat surface of the cycled aluminum foil anode wasobserved.5,13,16 Notably, when the cycled Al-n foil was folded, amossy interlayer was observed at the crack site (Figure 1b).The magnified SEM image of this cracked site (Figure 1c)shows numerous nanosized wire-shaped crystalline fragmentshiding under a uniform membrane. This first observed, skin-cover-sands-like morphology accounts for why the cycledaluminum foil anode appears as a plain surface with an absenceof dendrites. According to the reversible anodic reaction ofdissolution/deposition of metallic and crystalline Al species (eq1), we speculate that these fragments are aluminum dendritesbased on many research efforts on aluminum electro-deposition:21,24

Figure 1. Morphology of Al foil after 1000 cycles. SEM images of the (a) plain surface of cycled aluminum foil and (b) cracked site of the foldedcycled aluminum foil. (c) Magnified SEM images of the cracked site, exhibiting the aluminum dendrite hidden under the protective layer.

Figure 2. Cross-section SEM images of normal aluminum foil before and after cycling. SEM images of (a) normal aluminum foil before cycling and(b) after 10 000 cycles. Magnified cross-section SEM images of the normal aluminum foil after 10 000 cycles, exhibiting (c) an ultrathin protectivelayer with cubic dendrite and (d) the continuous plain surface of the protective layer. (e) Cross-section images of the cycled aluminum foil withelement mapping of (f) aluminum and (g) oxygen.

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+ ⇌ +− − −4Al Cl 3e Al 7AlCl2 7 4 (1)

To further study this interesting and first-witnessed phenom-enon, we investigated the cross-section SEM images of theuniform Al-n foils before being cycled (Figure 2a) and afterdifferent cycles (Figure S1). As the cycling proceeded, theuniformly dense Al-n foil gradually transferred into a porousstructure exactly from the direction facing the cathode andelectrolyte (Figure S1),28 and the proportion of dendritic areagradually increased. After 10 000 cycles in the aluminum−graphene full cell, the aluminum foil anode completely turnedinto a sandwich structure, as shown in Figure 2b with twoultrathin protective films with thickness lower than 50 nm(Figure 2c; Figure S2) homogeneously covering the highlyporous thick filled layer. Such a filled layer consisted ofexcessive wire-shaped aluminum dendrite with diameter around100 nm, as exhibited in Figure 2c. This form of aluminumdendrite is very similar to that of wire-shaped lithium dendriteformed during reversible stripping/plating.29 In the gentlysloping cross-section SEM images of the cycled Al-n foil anode,this ultrathin protective film exhibited a flat, clean, andcontinuous surface uniformly covering the porous interlayerand dendrites (Figure 2d), which consolidates the explanationsfor the plain and dendrite-free surface aforementioned. Thisgradual transformation of interlayer of aluminum metal anodeduring cycling is very similar to that morphology of cycledlithium metal anode in Li-ion batteries,23,29 affording dendritesthat compromise the safety of battery operation. However,unlike the brittle solid electrolyte interphase (SEI) that derivesfrom decomposition of electrolyte and then compactly coats onlithium metal anode,30 there exist nanosized interspacesbetween the protective film and porous interlayer of aluminummetal anode. This phenomenon reveals that this uniqueprotective film may natively exist in the commercial aluminumfoil, rather than like that SEI of lithium metal anode comingfrom electrolyte decomposition in the metal−electrolyteinterphase.30

To further study the component of this protective filmcovering the cycled aluminum foil, we conducted semi-

quantitative EDS mapping on the cross section of cycled Al-nfoil (Figures 2e−g; Figure S3). Only four elements wereobserved in this cycled Al-n anode: 78 wt % aluminum, 10.33wt % oxygen, and only 7 wt % carbon with 3 wt % chlorine,which may come from residual electrolyte and dichloro-methane. Although the aluminum species (Figure 2f) withother neglectable species (Figure S3) are uniformly distributedacross the aluminum foil, the oxygen species are mostlyconcentrated on the side corresponding to the protective layer(Figure 2g). This element distribution directly demonstratesthe constituent elements of such a cycled Al-n foil: purealuminum in the filled-layer (Figure S4), and aluminum oxideat the protective layer. The element component of surficialoxide film is supported by XPS spectra, indicating existence ofseveral elements: C, O, Al, Si, Cl, and N. These elementscorrespond well with the element species of the oxide film,residue electrolyte, dichloromethane, and glass-fiber separator.Impressively, the deconvoluted Al 2p spectrum is mainlycomposed of the 2p3/2 peak of Al2O3 at 74.2 eV. Meanwhile,peaks at 72.8 and 72 eV corresponding to Al0 become almostundetectable, especially when compared with Al-n foil beforebeing cycled (Figure S4a). This supports that the surficialprotective film is mostly composed of Al2O3, which wasassumed to be derived from the native oxide film that coated onaluminum foil. Thus, this also explains that the unevendistributed oxygen species in cycled Al-n (as revealed by EDSand XPS) should come from the surficial oxide film.To demonstrate the effect and origination of such a

protective aluminum oxide film, we inspected the CV curves(Figure 3c) and voltage profiles (Figure S5) of the symmetricaluminum−aluminum cell and cycle stability of the aluminum−graphene full cell. Two kinds of aluminum metal anodes wereutilized in cell tests: as-received Al-n that was exposed to air,and Al-r whose oxide film was removed beforehand. The CVcurves of symmetric cell exhibit a reversible anodic peak ataround 0.25 V (dissolution of aluminum) and cathodic peak at−0.25 V (deposition of aluminum) at a scan rate of 1 mV s−1,demonstrating the stable and reversible aluminum plating/

Figure 3. Analysis of the protective layer component of cycled aluminum foil and electrochemical behaviors. (a) Total XPS spectra and (b) Al 2pspectra. (c) CV curves of the aluminum−aluminum symmetric cell at different scan rates. (d) Cycling performance of aluminum−graphene full cellsusing Al-n or Al-r anodes; the capacity is based on mass of graphene cathode.

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stripping in the ionic liquid electrolyte.5,16,20,27 When the scanrate was increased to 10 mV s−1, these peaks exhibited smallchanges in position yet high increases in corresponding peakcurrent, suggesting an ideal high electronic-conducting anodematerial in principle. Stable voltage profiles of the Al-nsymmetric cell in Figure S5a exhibited a very small yet constanthysteresis at a current density of 1 mA cm−2. This smallhysteresis with flat voltage plateau can be easily attainedthroughout 400 h (400 cycles). However, the Al-r symmetriccell displayed a 100% higher hysteresis compared with that ofthe Al-n symmetric cell. After 240 h, a sudden voltage drop wasobserved for Al-r foil, suggesting an internal short circuit causedby aluminum dendrite penetration. Given that the unique highpower density advantage of aluminum−graphene battery isbased on performances of electrodes under higher currentdensities, the areal current density tested on aluminum anodewas increased to 5 mA cm−2 to detect the aluminum foil’scycling behavior at higher current density. Impressively, stablecycling beyond 1100 cycles was attained for the Al-n foil(Figure S5b) with flat and similar voltage plateau displayed in100th, 400th, and 1000th cycles (Figure S5c). In comparison,the Al-r foil counterparts displayed varying hysteresis andmultiple internal soft short circuit (Figure S5b) with fluctuatingvoltage profiles through these cycles (Figure S5d). Such ahigher overpotential can lead to worse dendrite growth, asreported by a recent reference, which causes a vicious circle:more dendrites lead to higher overpotential, and higheroverpotential leads to more dendrites.24 In the full cell

fabricated with Al-n anode and graphene cathode, a stablecapacity of ∼50 mAh g−1 based on defective graphene cathodeis delivered over 45 000 cycles (Figure 3d). Even though thefull cell using Al-r anode and the same graphene cathodeafforded similar specific capacity and stability within initial10 000 cycles, fast decay was observed in the following 10 000cycles with total cell failure at around 20 000 cycles. Notably,this phenomenon of fast decay from the 10 000th cycle and celltotal failure around the 20 000th cycle is commonly observed inmultiple full cells using Al-r anode. These differences inelectrochemical performance strongly support the vitalimportance of the surficial oxide film.The critical role of surficial oxide film can be further

supported by post-mortem SEM images of cycled Al-r foilanode. In comparison to those of Al-n foil, numerous aluminumdendrites can be directly observed from the top view of Al-ranode only after 1000 cycles (Figure 4a; Figure S6) without anyappearance of protective film. Such a difference in morphologydirectly proves that the surficial protective film originates fromthe native aluminum oxide film which was removed in Al-r foilbeforehand rather than from the decomposition of electrolyteshown in the SEI case of the lithium metal anode.30 Theabsence of innate surficial oxide film can also be confirmed bydifference in Al 2p spectra of Al-n and Al-r anodes (Figure S4).Cross-section SEM image of the Al-r anode after 1000 cycles(Figure 4b) exhibited severe mossy dendrite shooting out fromthe surface without any appearance of protective filmaforementioned. Without the protection of surficial protective

Figure 4. Analysis of the different behavior of Al-n and Al-r anodes. (a) Surficial SEM image of cycled Al-r after 1000 cycles. (b) Cross-section SEMimage of Al-r anode after 1000 cycles. Comparison of the (c) Al-n anode and (f) Al-r anode after 10 000 cycles. (d) Reaction of cycled Al-n anode(after 10 000 cycles) with methanol. (e) Magnified SEM image of aluminum dendrite.

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film, the dendrites can keep growing perpendicular to theanode surface without any suppression and may pierce theseparator to cause the cell short circuit safety hazard. MagnifiedSEM images on the dendrite clearly revealed the branchingshape of an aluminum dendrite (Figure 4e),21 which is quitesimilar to that of the lithium dendrite.29 The thermodynamicand dynamics of aluminum dendrite growth have been well-reported by previous reports on electrodeposition of aluminumspecies, revealing a visible perpendicular growth that isdependent on the overpotential, electrode material, and kineticparameters.24,31 As the cycling proceeds, this dendrite growthwill be more and more severe to induce higher overpotential24

and will eventually cause the cell short circuit just as exhibitedin Figure S5. The detailed dendrite growth mechanism is anopen question which needs more theoretical and experimentalstudies in the future. Element mapping on the cross section ofcycled Al-r foil displayed homogeneous element distribution(especially oxygen) across aluminum metal (Figure S7), whichdiffers from that of Al-n foil with oxygen concentrated on thesurficial film. Significantly, the Al-n anode maintains its integrityafter 45 000 cycles with less than 5% weight loss (Figure 4c). Itis worth noticing that the total amount of charge/dischargecapacity of this cell reached more than 3 Ah, which was 100times higher than the theoretic capacity of aluminum foil (10mg). This comparison consolidates the reversibility of thealuminum metal anode. The cycled Al-n foil can even react withanhydrous methanol and water to create suspected hydrogenbubbles (Figure 4d, Figure S8, and Videos S1 and S2), which isa unique property of highly reactive nanosized aluminumdendrite (Figure S8).32 By contrast, the Al-r anode seriouslycollapsed into powder-form aluminum debris with only few leftafter 10 000 cycles (Figure 4f) due to the missing oxide film,which can suppress dendrite formation and maintain itsintegrity. Notably, after 20 000 cycles, the Al-r anode totallycollapsed into black powder that could not be collected at all.These results demonstrate that the ultrathin native oxidationfilm33,34 leads to this distinction of stability between Al-n andAl-r anode and corresponding full cells.Even though the critical roles of aluminum oxide film in

suppressing dendrite growth and maintaining the integrity ofanode are confirmed above, a vital issue arises: how does theionic liquid electrolyte immerse through this dense and uniformyet nonionic-conducting aluminum oxidation film to react withinner aluminum metal? Hence, we inspect the SEM images ofAl-n anode after 10 000 cycles, displaying abundant cavities

with semitransparent film covering the inner holes (Figure 5a).It is worth noticing that these holes exist only under fissures inthe protective semitransparent film. The plain inner surface of afilled interlayer, which demonstrates absence of aluminumstripping/platting reaction, was observed under the continuousunbroken protective semitransparent film, as outlined bysquares. Magnified SEM image of the fissure is shown inFigure 5b. Nanosized aluminum dendrite can be clearlyobserved in the hole beneath the fissure, proving that thealuminum dendrite can be formed below the defect sites ofprotective semitransparent film. Elemental mapping was castedon this defect site, showing uniform distribution of aluminum(Figure 5c) and other species (Figure S9). However, theoxygen species were mostly concentrated in the region coveredwith such protective semitransparent film (Figure 5d). Thisuneven element distribution is consistent with previous resultsshown in Figure 2, suggesting that the semitransparent layer isjust the protective aluminum oxidation film aforementioned.Notably, as shown in Figure 5a, aluminum dendrites wereobserved only under the defect sites of protective aluminumoxidation film, which were the evidence for reaction sites ofaluminum stripping/plating.35 These phenomena demonstratethat only when defects existed in the surficial protectivealuminum oxidation film can the internal aluminum beneaththis fissure become reactive, which is in accordance with theenhancement of electroactivity in the defect sites of nativealuminum oxide film reported.36 Accordingly, a modeling ofaluminum metal anode can be built as exhibited in Figure 5e tospeculate the exact role of aluminum oxide film and reactionmechanism of Al-n anode:35,37 the electrolyte can infiltrate onlythrough defect sites in the native aluminum oxide film to reactwith internal aluminum metal at the metal/oxide interface,affording fewer nucleation sites than Al-r without such surficialoxide film. Owing to the defects in oxide film, Al-n anode candeliver decent electrochemical performance under surfacecoating of such an electron and ion insulative oxide film(Figure S10). This inert aluminum oxide film with high Young’smodulus (>100 GPa)23,38 can effectively suppress the dendritegrowth and inhibit further disintegration of the anode byconfined reaction space, restricted nucleation site, and nearlyfull interfacial protection. In contrast, the Al-r foil anode asmodeled in Figure 5f does not possess such an effectiveprotective film. Therefore, aluminum dendrites are developedall over the surface without limitation, leading to a ruggedsurface of Al metal anode and further cell short circuit. After

Figure 5. Analysis of the mechanism of the aluminum foil protected by the oxide film. (a) SEM images of Al-n anode after 10 000 cycles. (b)Magnified SEM images of Al-n anode after 10 000 cycles, exhibiting a hole in the protective layer with most dendrite appearing beneath this holewith element mapping of (c) aluminum and (d) oxygen. Models of cycled (e) Al-n metal and (f) Al-r metal.

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several cycles, dead aluminum dendrite can be easily detachedfrom original aluminum foil due to unrestricted volumevariation and inhomogeneous dissolution of dendrites, resultingin active material loss and further electrode disintegration(Figure 4d). This unique model of aluminum metal anodeaccords with the experimental data aforementioned and canrationalize such a stable cycling of metallic aluminum anodewell.

■ CONCLUSIONSIn summary, we demonstrate Al dendrite in the aluminummetal anode of AIBs and the critical role of native surficialaluminum oxide film in suppressing dendrites and stabilizingthe anode. Rather than suspected dendrite-free behavior, thealuminum dendrites do exist, yet they are confined beneath theprotective aluminum oxide layer. Through restricted nucleationsites and nearly full interfacial protection, the native aluminumoxide film can effectively suppress the growth of aluminumdendrite and prevent the disintegration of aluminum foil anodeduring repeated anodic aluminum plating/stripping reaction,affording an almost intact aluminum foil with flat surface aftercycling. In addition, the defect sites in the aluminum oxide filmenable the penetration of electrolyte and the internal reactionof aluminum plating/stripping in the oxide/metal interface,which accounts for how the aluminum metal acts as a reactiveanode under such a nearly full interfacial protection by thenative aluminum oxide film. Benefiting from this protectivefilm, the Al-n metal anode can attain stable cycling withoutshort circuit or fluctuating voltage profiles, and the aluminum−graphene full cell achieved a stable cycling over 45 000 cycles.This unique dendrite issue of aluminum metal anode indicates afundamental insight into the aluminum foil anode of AIBs,prompting both practical applications and further study onaluminum metal anodes and AIBs.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b07024.

Cross-section SEM images of Al-n foil after differentcycles, magnified cross-section SEM images of theultrathin protective layer covering Al-n foil anode after10 000 cycles, element distribution of cycled Al-n foilanode, deconvoluted Al 2p spectrum and XRD spectrumof Al-n (Al-r) metal before and after cycling, galvano-static cycling of symmetric aluminum foil, SEM images ofAl-r metal anode after 1000 cycles, element distributionof cycled Al-r foil anode, photograph of cycled Al-n metalreacting with methanol and water, and element mappingof carbon and chlorine corresponding to Figure 5,comparison of batteries’ performances using Al-n or Al-ranodes (PDF)Reaction of cycled aluminum metal with methanol(MPG)Reaction of cycled aluminum metal with water (MPG)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: chaogao@zju.edu.cn.ORCIDChao Gao: 0000-0002-3893-7224

FundingThis work was supported by the National Natural ScienceFoundation of China (Grants 21325417, 51533008, and51603183), National Key R&D Program of China (Grant2016YFA0200200), and Fundamental Research Funds for theCentra l Univers i t ies (Grants 2017QNA4036 and2017XZZX008-006).

NotesThe authors declare no competing financial interest.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.7b07024ACS Appl. Mater. Interfaces 2017, 9, 22628−22634

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