Fabrication of ZnO Nanowires Arrays by Anodization and High-Vacuum Die Casting Technique, and Their Piezoelectric Properties

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1 sensors Article Fabrication Nanowires Arrays by Anodization High-Vacuum Die Casting Technique, Their Piezoelectric Properties Chin-Guo Kuo 1, Ho Chang 2, * Jian-Hao Wang 2 1 Department Industrial Education, National Taiwan Normal University, Taipei 6, Taiwan; chinguo7@yahoo.com.tw 2 Graduate Institute Manufacturing Technology, National Taipei University Technology, Taipei 608, Taiwan; t @ntut.edu.tw * Correspondence: f381@ntut.edu.tw; Tel.: ; Fax: Academic Editor: Teen-Hang Meen Received: 19 January 2016; Accepted: 22 March 2016; Published: 24 March 2016 Abstract: In this investigation, anodic aluminum oxide (AAO) with arrayed regularly arranged nanopores is used as a in high-vacuum die casting molten zinc metal (Zn) into nanopores. The proposed technique yields arrayed Zn with an aspect ratio over 600. After annealing, arrayed zinc oxide () are obtained. Varying anodizing time yields AAO s with es approximately 50 µm, 60 µm, 70 µm can be used in fabrication three lengths with high aspect ratios. Experimental results reveal a longer nanowire generates a greater measured piezoelectric current. The are fabricated an alumina are for 7 h produce higher piezoelectric current up to 69 pa. Keywords: ; anodic aluminum oxide (AAO); vacuum die casting 1. Introduction Zinc oxide () is an n-type II-VI semiconductor group material with a wurtzite structure in a hexagonal crystal system. Since it is symmetric has no center symmetry, this structure has favorable piezoelectric properties [1]. As components are miniaturized, piezoelectric materials have become nanosized. In recent years, (NWs) have been used in nanogeneration devices [2,3]. In relevant works, most representative device this kind is piezoelectric nanogenerator developed by a research team led by Wang [4]. First, an atomic force microscope (AFM) used as a probe to apply stress to to make m produce strain, n piezoelectric current measured. This furr employed to develop a nanogenerator. Xue studied response a nanogenerator (NG) to gas environment in which it is placed. Xue s experimental results indicated output a piezoelectric nanogenerator (NG) fabricated nanowire arrays is largely influenced by density surface charge carriers at nanowire surfaces [5]. Lin developed Pd nanoparticles were uniformly loaded on whole surfaces nanowire arrays a simple hydrormal method. The piezoelectric output generated by Pd/ nanoarray nanogenerator acted not only as a power source but also a response signal to ethanol at room temperature [6]. Fu fabricated a ZnSnO 3 / nanowire piezo-nanogenerator as a self-powered gas sensor with high sensitivity, selectivity, reliability to detect liquefied petroleum gas at room temperature. The results indicated sensitivity self-powered gas sensor to 8000 ppm liquefied petroleum gas up to 83.23, limit detection 600 ppm [7]. Zeng utilized a low-temperature hydrormal method to fabricate vertically aligned Sensors 2016, 16, 431; doi:.3390/s

2 Sensors 2016, 16, were grown on graphene oxide (GO) films. The results indicated graphene oxide layer facilitated were grown vertical ongrowth graphene oxide (GO) NWs films. improved The resultsir indicated crystal quality graphene [8]. Lin oxide used a layer solution-free, facilitatedcatalyst-free, verticalvapor-phase growth growth NWs method improved to synsize ir crystal nanorod quality arrays [8]. Lin on used Al-doped a solution-free, (AZO) catalyst-free, films. The experimental vapor-phase results growth method Lin revealed to synsize electrical nanorod performance arrays on Al-doped transparent (AZO) conductive films. Theoxide experimental films results not degraded Lin revealed by growth electrical nanorod performance arrays, transparent nanorods were conductive highly crystalline oxide films [9]. Tian notperformed degraded pure by Zn growth chemical nanorod vapor deposition arrays, on an nanorods anodic aluminum were highly oxide crystalline substrate [9]. Tian performed pure Zn chemical vapor deposition on an anodic aluminum oxide substrate at 950 C at 950 C to form develop multilayer comb-like zinc oxide nanostructure. The experimental results to form Tian develop reveal multilayer growth comb-like zinc oxide nanostructures nanostructure. is The controlled experimental by vapor-solid results Tian (VS) reveal growth mechanism growth is a diffusion-limited nanostructuresprocess is controlled []. by vapor-solid To fabricate (VS) growth a one-dimensional mechanism nanostructure, is a diffusion-limited Zhang et al. process successfully []. placed bismuth (Bi) in a vacuum To fabricate chamber afor one-dimensional heating melting, nanostructure, cast Zhang molten et al. successfully Bi into alumina placed bismuth (Bi) ahigh-pressure vacuum chamber gas [11,12]. for heating A high temperature melting, cast used to molten melt Bimetal. into The alumina molten metal material high-pressure cast into gas nano [11,12]. A highby temperature pressurization to used fabricate to meltarrayed metal.. The molten In metal fabrication material cast, into nano control variables pressurization were pressure to fabricate value, heating arrayedtemperature,. In properties fabrication liquid, metal, control applied variables force. were However, pressure pressure value, heating temperature, high-pressure gas properties limited by liquid a gas metal, compressor. To applied solve force. problems However, pressure were caused by high-pressure gas injection, gasmechanically limited by powered a gas compressor. hydraulic equipment To solve problems used to apply were caused additional by gaspressure injection, in mechanically a newly developed powered hydraulic high-vacuum equipment die casting technique used to apply combined additionaltraditional pressure incasting a newly with developed a nano-technique high-vacuum [13]. die casting Anodization technique combined used to fabricate traditional porous casting anodic withaluminum a nano-technique oxide (AAO) [13]. s. The pore size Anodization used controlled to fabricate porous various anodic solutions. aluminum Accordingly, oxide (AAO) highly s. regularly The arrayed pore size nanopores were formed controlled [14]. The depth various pores solutions. determined Accordingly, by highly anodizing regularlyduration. arrayed nanopores Then high-vacuum were formed die casting [14]. The technique depth used pores to cast determined material by into anodizing nanopores Then, high-vacuum thus die to fabricate casting technique used different to cast lengths. material The purpose into nanopores this work is to, fabricate thus nanowire to fabricate with a diameter different 80 nm inside lengths. nanopores The purpose an this AAO work is to fabricate to study nanowire relationship with a diameter between 80length nm inside nanopores nanowire AAO piezoelectric properties to study relationship material between with a one-dimensional length nanostructure. nanowire The results piezoelectric this research properties may improve material our with understing a one-dimensional material nanostructure. on The nanoscale. results this research may improve our understing material on nanoscale. 2. Experimental Details 2. Experimental Details In experiment herein, porous AAO film used as a, on which zinc foil In experiment herein, porous AAO film used as a, on which zinc foil placed. placed. The heated until molten. Normal pressure applied to cast molten zinc The heated until molten. Normal pressure applied to cast molten zinc into into nanopores. Atmospheric annealing performed to transform zinc nanopores. Atmospheric annealing performed to transform zinc into. Finally, part AAO nano removed to expose into. Finally, part AAO nano removed to expose.. 1 shows a flow chart experiment. 1 shows a flow chart experiment. 1. Experimental process. AAO: anodic aluminum oxide. In 1995, Masuda et al. discovered self-assembling porous alumina film, developed hexagonally arrayed porous alumina alumina [15]. [15]. They They used used a two-step a two-step anodization anodization method method to grow to alumina grow film. alumina Aluminum film. Aluminum foil with foil awith purity a purity 99.7% 99.7% used used as as substrate. substrate. HClO HClO4, 4, CC2H5OH, 2 H 5 HOCH2CH2OC4H9 were mixed uniformly in a ratio 3:14:3 to form an electrolytic polishing

3 Sensors Sensors 2016, 2016, 16, 16, Sensors 2016, 16, solution. With pure aluminum foil as anode graphite foil as cathode, electrolytic HOCH polishing 2 CH 2 OC 4 H 9 foil were mixed performed, uniformly completely in a ratio flattening, 3:14:3 to form lustering, an electrolytic smoothing polishing its solution. surface, solution. With pure aluminum foil as anode graphite foil as cathode, electrolytic With favoring pure aluminum fabrication foil as anode AAO film graphite with very foilregularly as cathode, arrayed electrolytic nanopores. polishing Thereafter, 0.3 foil M polishing foil performed, completely flattening, lustering, smoothing its surface, oxalic performed, acid solution completely used flattening, two-phase lustering, anodization. smoothing With itsaluminum surface, favoring foil as fabrication anode favoring fabrication AAO film with very regularly arrayed nanopores. Thereafter, 0.3 M graphite AAOfoil film as withcathode, very regularly 40 V voltage arrayed nanopores. applied for Thereafter, 1 h to perform 0.3 Mphase-1 oxalic acid anodization. solutionthen, oxalic acid solution used in two-phase anodization. With aluminum foil as anode used 6% phosphoric in two-phase acid anodization. solution With added aluminum to 2% foil chromic as acid anode solution, graphite foil resulting as cathode, solution 40 V graphite foil as cathode, 40 V voltage applied for 1 h to perform phase-1 anodization. Then, voltage used to remove applied for AAO 1 hfilm to perform phase-1 grown anodization. in phase-1 at Then, 60 C. 6% Finally, phosphoric phase-2 acid anodization solution 6% phosphoric acid solution added to 2% chromic acid solution, resulting solution added performed. to 2% A chromic second acid anodization solution, resulting solution used to remove AAO film used to remove AAO film grown in phase-1 at 60 C. for 5, 6, 7 h yielded films various es. As presented grown in phase-1 at 60 C. Finally, phase-2 anodization in 2, nanopores in Finally, obtained phase-2aao anodization were performed. very regularly A second arrayed anodization with a performed. A second anodization for 5, 6, 7 h yielded films various es. As presented for complete 5, 6, structure, 7 h yielded films all pores various had a size es. 80 nm. As presented in 2, nanopores in in 2, nanopores in obtained AAO were very regularly arrayed with a obtained AAO were very regularly arrayed with a complete structure, all pores had complete structure, all pores had a size 80 nm. size 80 nm. 2. SEM image alumina with pores size 80 nm. 2. SEM image alumina with pores size 80 nm. The nanopores in AAO fabricated in experiment are highly regularly arrayed. The diameter pores is 80 nm, tube wall increases with time. The in are highly arrayed. helps The in nanopores fabricating in, AAOsupporting fabricated good in regularity. experiment The are AAO highly regularly arrayed. zinc film were The diameter placed in die pores cast ismold, 80 nm, as presented in 3. The tubemold wall increases depressurized with time. to The a vacuum helps in fabricating, supporting good regularity. The AAO zinc film were placed 3 torr, heated at 750 C for 60 min. Then, a hydraulic force (kgf/cm in die cast mold, as as presented in in The mold depressurized 2 ) applied to cast to a vacuum molten zinc into nanopores 3 3 torr, torr, heatedat at 750 C C AAO. A period condensation yields zinc for 60 min. Then, a hydraulic force (kgf/cm 2 ) applied to cast. molten zinc zinc into into nanopores nanopores AAO AAO.. A period A period condensation condensation yields zincyields. zinc. 3. Die casting mold. 3. Die casting mold. Let 3. Die casting mold. applied pressure liquid metal enters AAO be be proportional to to surface surface tension tension liquid metal. The The surface tension zinc in liquid state at 600 C C is is Let applied pressure liquid metal enters AAO be proportional to 787 dyne/cm [16]. A hydraulic force applied to to overcome surface tension liquid; surface tension liquid metal. The surface tension zinc in liquid state at 600 C is pressure liquid enters a nanotube calculated Equation (1), (1), where where F denotes F denotes 787 dyne/cm [16]. hydraulic force applied to overcome surface tension liquid; normal normal force; force; A is A is surface surface area area sample; sample; γ is γ issurface surface tension tension liquid; liquid; θ is θcontact is pressure liquid enters a nanotube calculated Equation (1), where F denotes contact angle between angle between liquid liquid solid, solid, r is r isdiameter diameter microtubules. As measured As measured by a normal force; A is surface area sample; γ is surface tension liquid; θ is contact by contact a contact angle angle analyzer, analyzer, θ θaao AAO The The diameter diameter AAO AAO tube tube 80 nm. 80 nm. The angle between liquid solid, r is diameter microtubules. As measured by a The surface surface area area sample cm contact angle analyzer, θ AAO Therefore, additionally applied force zinc liquid The diameter AAO tube 80 nm. The entering critical side AAO 3.16 ˆ 8 surface area sample 3.14 cm 2. Therefore, dyne. additionally applied force zinc liquid entering critical side AAO dyne.

4 Sensors 2016, 16, Sensors 2016, 16, P P = F{A F/A = 2γ 2γ(cosθ/r) pcosθ{rq (1) The zinc, following die die casting, were were placed placed in anin atmospheric an atmospheric heat treatment heat treatment furnace. furnace. The temperature The temperature furnace furnace increased increased to 300 C to 300 maintained C maintained for 36 h to carry for 36 out h to oxidization carry out oxidization to completely to completely transform transform zinc into zinc zinc oxide into zinc oxide [17]. A part [17]. A AAO part AAO removed removed 0.1 M sodium hydroxide 0.1 M sodium (NaOH) hydroxide solution (NaOH) to expose solution. to expose Since. part Since part remains, remains, maintain ir verticality, maintain ir verticality, overall structure overall structure array has enough strength array has to enough facilitatestrength subsequent to facilitate measurement subsequent its piezoelectric measurement properties. its piezoelectric Finally, deionized properties. waterfinally, used deionized to perform water ultrasonic used cleaning to perform to ultrasonic remove cleaning residues to after remove reaction residues after AAO reaction wi NaOH. AAO Bake drying with NaOH. performed Bake drying on a heating performed platform. on This a heating concludes platform. fabrication This concludes fabrication nanowire piezoelectric nanowire component. piezoelectric component. This paper uses SEM to observe AAO, an X-ray diffractometer (XRD) to analyze crystal crystal structure.. This study This verifies study verifies if are completely are completely transformed transformed into, into, n crystal n direction crystal direction three lengths three can lengths be observed. can be Finally, observed. a conductive Finally, a conductive atomic force atomic microscope force microscope (C-AFM) is(c-afm) used to scan is used to scan in contact in mode contact mode measure measure piezoelectric piezoelectric current. The current. length, The width, length, width, height height AFM cantilever AFM cantilever are 160 µm, are µm, μm, μm, µm, respectively. 4.6 μm, respectively. The height The height radius radius probe are probe 14 µm are 14 7 μm nm, respectively. 7 nm, respectively Results Discussion The surface morphology is isobserved by byscanning electron microscopy (SEM). 4 shows SEM images a nanowire is cast into an AAO SEM SEMimages images are arecast castinto intoan anaao : top topview; lateral lateralview. 5a shows SEM front image partly removed AAO. 5b displays 5a shows SEM front image partly removed AAO. 5b displays SEM image sample is slanted 30. In figures, well distribution SEM image sample is slanted 30. In figures, well distribution array array can be observed. The are found to be very densely arrayed, can be observed. The are found to be very densely arrayed, each exhibits good each exhibits good verticality. The SEM images in 5a,b reveal diameter verticality. The SEM images in 5a,b reveal diameter prepared is prepared is 80 nm se are primarily well aligned on bottom 80 nm se are primarily well aligned on bottom AOO. 5c AOO. 5c presents energy dispersive spectrometer (EDS) analytic chart. The presents energy dispersive spectrometer (EDS) analytic chart. The area EDS analysis is indicated area EDS analysis is indicated by blue square in 5a. Table 1 presents constituent by blue square in 5a. Table 1 presents constituent elements fabricated. elements fabricated. The table reveals atmospheric annealing can transform The table reveals atmospheric annealing can transform zinc into. Additionally, zinc into. Additionally, to elucidate crystalline quality prepared to elucidate crystalline quality prepared, 6 presents a TEM image, 6 presents a TEM image re. The prepared are single-crystal re. The prepared are single-crystal with a wurtzite structure a <0001> with a wurtzite structure a <0001> growth direction. growth direction.

5 Sensors 2016, 16, 431 Sensors 2016, 16, 431 Sensors 2016, 16, (c) (c) 5. SEM images : top view; lateral view, (c) EDS pattern SEM SEM images images : : top top view; view; lateral lateral view, view, (c) (c) EDS EDS pattern. pattern. Table 1. EDS analytic composition nanowire. Table 1. Table 1. EDS EDS analytic analytic composition composition nanowire. nanowire. Element Element OElement K OZnKLO K Zn LZn L Total TotalTotal Weight % Weight % Weight % Atomic % Atomic Atomic % % TEM image prepared. 6. TEM image prepared. 6. TEMs image prepared. As shown in 7, AAO were for 5 8 h had es 49.4 m, 59.5 m, 68.1 m, 72.6 m. The each determined by As shown inin 7,anodization. AAO s were for 5 8 h had 49.4 µm, shown 7, AAO s were for 5 8 es h had es duration second 8 plots relationship between 59.5 µm, 68.1 µm, 72.6 µm. The each determined by duration 49.4 m, 59.5 m, 68.1 m, 72.6 m. The each determined by anodizing duration. When anodizing time 5 7 h, increased at anodization. plots relationship duration 9 μm/h, second anodization. 8 plots between relationship between a second rate when8 anodizing time 7 8 h, increased at a rate anodizing duration. anodizing time 5 7 h,a increased at acould rate because anodizing duration. When anodizing time 5 7 h, increased at 4 5 μm/h, whenwhen reached certain value, electrolyte 9 µm/h, when anodizing time 7 8rate h, a rate at4 5 µm/h, easily bottom when pores, so growth reduced. a rate reach 9 μm/h, anodizing time 7 8 h, increased at increased a rate because when reached a certain value, electrolyte easily could reach 4 5 μm/h, because when reached a certain value, could electrolyte bottom bottom pores, so pores, growth reduced. easily reach sorate growth rate reduced.

6 Sensors 2016, 16, Sensors 2016, 16, 16, (c) (d) SEM images cross-section alumina s were for 5 8 h: h: 5 h, h, 6 h, h, (c) 7 h, h, (d) 8 h. h. Thickness ( m) ( m) Time (h) Relationship between alumina anodizing duration. 8. Relationship between alumina anodizing duration. 9 presents XRD diagrams were obtained at at 0.02 intervals diffraction angle 2θ from 30 to 9 70, presents following XRD diagrams XRD measurement were obtained at 0.02 intervals diffraction were made angle 2θ from 30 AAO to 70, following XRD measurement for 5 7 h. When diffraction angle 2θ were made approximately AAO 34.3, all three for 5 7 lengths h. When yielded a diffraction very strong angle diffraction 2θ approximately peak. Analysis 34.3, XRD chart a comparison all three lengths with yielded JCPDS database a very strong revealed diffraction peak. peak Analysis value generated XRD by chart a comparison (002) with side. JCPDS Therefore, database revealed peak were value fabricated generated in this by work exhibit preferentially (002) side. c-axis-oriented Therefore, growth, peak were strength fabricated tends in to this be proportional work exhibit to preferentially length c-axis-oriented nanowire. The growth, chart also reveals peak strength Zn tends to be proportional have undergone to heat length treatment nanowire. at at a fixed The temperature chart also reveals 300 C for 36 Zn h are completely have oxidized undergone heat thus treatment transformed at a into fixed temperature. 300 C for exhibits 36 h are non-ferroelectric completely oxidized piezoelectricity. thus transformed Therefore, into it it cannot. generate piezoelectricity by polarization, exhibits non-ferroelectric only a structure piezoelectricity. grows Therefore, along it c-axis cannot is generate is piezoelectric. piezoelectricity Therefore, by polarization, preferentially c-axis-oriented only a structure is is just grows highly along suitable c-axis for fabricating is piezoelectric. piezoelectric Therefore, components. preferentially c-axis-oriented is just highly suitable for fabricating piezoelectric components.

7 Sensors 2016, 16, 16, Sensors Sensors 2016, 2016, 16, Intensity (a.u.) (7 h) h) (7 (6 h) h) (6 (5 h) h) ( Theta (deg.) (deg.) 22 Theta XRD patterns were fabricated alumina XRD XRD patterns were were fabricated fabricated alumina alumina 9. patterns for 5 7 h. for for h. h. AFM in in contact mode used usedto measure surface surface morphology over 5 µm 5 µm areas contact mode mode used totomeasure measure surface morphology over μm ˆ μm areas morphology over 55 μm 55 μm areas surfaces were werefabricated fabricatedby by above above experimental experimental method. method. As As shown shown in surfaces were fabricated by above experimental method. As in surfaces,, surface surface morphology morphology is is very very densely densely arrayed, arrayed, each each nanowire nanowire exhibits exhibits good good verticality. verticality. verticality... 3D 3D diagram surface morphology morphology,, obtained obtained atomic atomic force force. 3D diagram diagram surface surface morphology, obtained atomic force microscopy (AFM). microscopy (AFM). microscopy (AFM). Conductive AFM AFM used used to to measure measure piezoelectric piezoelectric currents currents three three Conductive Conductive AFM used to measure piezoelectric currents three lengths. Through Through probe, probe, stress stress applied applied to to deform deform.. The The piezoelectric piezoelectric current current lengths. lengths. Through probe, stress applied to deform. The piezoelectric current generated generated on on aa certain certain area area each each nanowire nanowire measured. measured. When When aa current current generated generated generated on a certain area each nanowire measured. When a current generated on on aa surface, surface, probe probe received received aa signal. signal. An An image image current current distribution distribution on on scanned scanned area area on a surface, probe received a signal. An image current distribution on scanned area obtained. obtained. The The current/voltage current/voltage curve curve (I V (I V curve) curve) at at aa particular particular point point obtained. obtained. 11a c 11a c plot plot obtained. The current/voltage curve (I V curve) at a particular point obtained. 11a c plot piezoelectric currents on were fabricated alumina piezoelectric currents on were fabricated alumina piezoelectric currents on were fabricated alumina had had been been for for h, h, obtained obtained C-AFM. C-AFM. As As shown shown in in figures, figures, piezoelectric piezoelectric had been for 5 7 h, obtained C-AFM. As shown in figures, piezoelectric current current increased increased with with length length nanowire. nanowire. The The nanowire nanowire with with aa length length approximately approximately current increased with length nanowire. The nanowire with a length approximately 70 µm 70 μm μm fabricated fabricated after after anodization anodization for for 77 hh generates generates aa current current pa. pa. Nanowires Nanowires cause cause aa 70 fabricated after anodization for 7 h generates a current 69 pa. Nanowires cause a deflection a deflection aa probe probe is is applied applied to to stress stress m m under under scanning scanning process. process. As As length length deflection probe is applied to stress m under scanning process. As length increases, increases, increases, probability probability aa deflection deflection increases, increases, larger larger piezoelectric piezoelectric currents currents are are probability a deflection increases, larger piezoelectric currents are measured. measured. measured.

8 Sensors 2016, 2016, 16, 16, Sensors Sensors 2016, 16, (c)(c) Piezoelectric current diagrams,fabricated fabricated alumina alumina Piezoelectric current diagrams, fabricated alumina 11. Piezoelectric current diagrams, for 5 7 h, measured conductive atomic force microscopy (C-AFM): h, for for measured conductive atomic microscopy (C-AFM): h, h, measured conductive atomic forceforce microscopy (C-AFM): 5 h, 56 h, h,h. 6(c) h,67 (c) (c) 7 h.7 h. plotscurrent/voltage current/voltage curve curve measured measured a plots 12 plotsprobe current/voltage curve Schottky measured a platinum-plated serving as a metal electrode. Since contact exists between platinum-plated probe serving as a metal electrode. Since Schottky contact exists between platinum-plated probe serving as a metal electrode. Since Schottky contact exists between metal electrodes, current/voltage current/voltagecurve curve has has an an asymmetric asymmetric metal electrodes, metal electrodes, current/voltage curve has an asymmetric exhibits a diode.therefore, Therefore,a apiezoelectric piezoelectriccomponent component is is fabricated fabricated exhibits a diode. generates exhibits a diode. Therefore, a piezoelectric component is fabricated from a direct current. from generates a direct current. from generates a direct current. Current (pa) Current (pa) Voltage (V) Voltage (V) Current/voltage properties, measured a platinum-plated probe 12. properties 12. Current/voltage Current/voltage properties,, measured measured aa platinum-plated platinum-plated probe probe serves as a metal electrode. serves serves as as aa metal metal electrode. electrode.

9 Sensors 2016, 16, Conclusions In this work, AAO s were made, formation nanopores controlled, a high-vacuum die casting technique used to cast zinc into nanopores AAO. Zinc transformed into by atmospheric heat treatment, AAO n removed to expose. Microstructural analysis carried out observations made. Finally, piezoelectric current generated by measured C-AFM. The results this work are summarized as follows: 1. An aluminum with a purity 99.7% to produce an AAO. The nanopores in were highly regularly arrayed had a high aspect ratio. Finally, process parameters were optimized to minimize cost required consumable materials. 2. The stress associated with capillary phenomenon in die casting calculated to obtain normal force required to cast molten zinc metal into nanopores. The pressure adjusted by controller die casting machine. A hydraulic force used to cast molten zinc into AAO. After atmospheric heat treatment, arrayed were obtained. 3. The were fabricated AAO were very dense, had an aspect ratio over 600, were well arrayed, exhibited good verticality. 4. The arrayed were fabricated herein exhibited preferentially c-axis-oriented growth. The (002) peak strength proportional to length nanowire. 5. As observed from test piezoelectric properties, elucidated C-AFM, longer generated a greater measured piezoelectric current. Among se, those fabricated alumina had been for 7 h generated greatest piezoelectric current 69 pa. Acknowledgments: The authors would like to thank Ministry Science Technology Republic China, Taiwan, for financially supporting this research under Contract No. NSC E Author Contributions: Chin-Guo Kuo is a consultant to research supervised entire research work. Ho Chang is a main writer manuscript advised experiments. Jian-Hao Wang performed experiments assisted research. Conflicts Interest: The authors declare no conflict interest. References 1. Wang, R.; King, L.H.; Sleight, W. Highly conducting transparent thin films based on zinc oxide. J. Mater. Res. 1996, 11, [CrossRef] 2. Gao, Y.F.; Wang, Z.L. Electrostatic potential in a bent piezoelectric nanowire. The fundamental ory nanogenerator nanopiezotronics. Nano Lett. 2007, 7, [CrossRef] [PubMed] 3. Gao, P.G.; Song, J.H.; Lin, J.; Wang, Z.L. Nanowire piezoelectric nanogenerators on plastic substrates as flexible power sources for nanodevices. Adv. Mater. 2007, 19, [CrossRef] 4. Wang, Z.L.; Song, J. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006, 312, [CrossRef] [PubMed] 5. Xue, X.; Nie, Y.; He, B.; Xing, L.; Zhang, Y.; Wang, Z.H. Surface free-carrier screening effect on output a nanowire nanogenerator its potential as a self-powered active gas sensor. Nanotechnology 2013, 24. [CrossRef] [PubMed] 6. Lin, Y.; Deng, P.; Nie, Y.; Hu, Y.; Xing, L.; Zhang, Y.; Xue, X. Room-temperature self-powered ethanol sensing a Pd/ nanoarray nanogenerator driven by human finger movement. Nanoscale 2014, 6, [CrossRef] [PubMed] 7. Fu, Y.; Nie, Y.; Zhao, Y.; Wang, P.; Xing, L.; Zhang, Y.; Xue, X. Detecting liquefied petroleum gas (LPG) at room temperature ZnSnO3/ nanowire piezo-nanogenerator as self-powered gas sensor. ACS Appl. Mater. Interfaces 2015, 7, [CrossRef] [PubMed] 8. Zeng, H.; Cao, Y.; Xie, S.; Yang, J.; Tang, Z.; Wang, X.; Sun, L. Synsis, optical electrochemical properties /graphene oxide heterostructures. Nanoscale Res. Lett. 2013, 8. [CrossRef] [PubMed]

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