sensors Article

Fabrication of ZnO Nanowires Arrays by Anodization and High-Vacuum Die Casting Technique, and Their Piezoelectric Properties Chin-Guo Kuo 1 , Ho Chang 2, * and Jian-Hao Wang 2 1 2

*

Department of Industrial Education, National Taiwan Normal University, Taipei 10610, Taiwan; [email protected] Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 10608, Taiwan; [email protected] Correspondence: [email protected]; Tel.: +886-2-27712171; Fax: +886-2-27317191

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 and regularly arranged nanopores is used as a template in the high-vacuum die casting of molten zinc metal (Zn) into the nanopores. The proposed technique yields arrayed Zn nanowires with an aspect ratio of over 600. After annealing, arrayed zinc oxide (ZnO) nanowires are obtained. Varying the anodizing time yields AAO templates with thicknesses of approximately 50 µm, 60 µm, and 70 µm that can be used in the fabrication of nanowires of three lengths with high aspect ratios. Experimental results reveal that a longer nanowire generates a greater measured piezoelectric current. The ZnO nanowires that are fabricated using an alumina template are anodized for 7 h and produce higher piezoelectric current of up to 69 pA. Keywords: ZnO nanowires; anodic aluminum oxide (AAO); vacuum die casting

1. Introduction Zinc oxide (ZnO) is an n-type II-VI semiconductor group material with a wurtzite structure in a hexagonal crystal system. Since it is symmetric and has no center of symmetry, this structure has favorable piezoelectric properties [1]. As components are miniaturized, piezoelectric materials have become nanosized. In recent years, ZnO nanowires (NWs) have been used in nanogeneration devices [2,3]. In relevant works, the most representative device of this kind is the piezoelectric nanogenerator that was developed by a research team that was led by Wang [4]. First, an atomic force microscope (AFM) was used as a probe to apply stress to ZnO nanowires to make them produce strain, and then the piezoelectric current was measured. This property was further employed to develop a nanogenerator. Xue studied the response of a nanogenerator (NG) to the gas environment in which it is placed. Xue’s experimental results indicated that the output of a piezoelectric nanogenerator (NG) that was fabricated using ZnO nanowire arrays is largely influenced by the density of the surface charge carriers at the nanowire surfaces [5]. Lin developed Pd nanoparticles that were uniformly loaded on the whole surfaces of ZnO nanowire arrays using a simple hydrothermal method. The piezoelectric output that was generated by the Pd/ZnO nanoarray nanogenerator acted not only as a power source but also a response signal to ethanol at room temperature [6]. Fu fabricated a ZnSnO3 /ZnO nanowire piezo-nanogenerator as a self-powered gas sensor with high sensitivity, selectivity, and reliability to detect liquefied petroleum gas at room temperature. The results indicated that the sensitivity of the self-powered gas sensor to 8000 ppm liquefied petroleum gas was up to 83.23, and the limit of detection was 600 ppm [7]. Zeng utilized a low-temperature hydrothermal method to fabricate vertically aligned

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nanowires that were grown on graphene oxide (GO) films. The results indicated that the graphene ZnO that were on growth graphene The resultstheir indicated that the graphene oxidenanowires layer facilitated thegrown vertical ofoxide ZnO (GO) NWsfilms. and improved crystal quality [8]. Lin oxide layer facilitated the vertical growth of ZnO NWs and improved their crystal quality [8]. Lin used a solution-free, catalyst-free, vapor-phase growth method to synthesize ZnO nanorod arrays on used a solution-free, catalyst-free, vapor-phase growth method to synthesize ZnO nanorod arrays on Al-doped ZnO (AZO) films. The experimental results of Lin revealed that the electrical performance Al-doped ZnO (AZO) films. Theoxide experimental of Lin revealed the electrical performance of of the transparent conductive films wasresults not degraded by thethat growth of nanorod arrays, and the transparent conductive oxide films was not degraded by the growth of nanorod arrays, and the the nanorods were highly crystalline [9]. Tian performed pure Zn chemical vapor deposition on an nanorods were highly crystalline [9]. Tian performed pure develop Zn chemical vapor deposition an anodic anodic aluminum oxide substrate at 950 °C to form multilayer comb-likeonzinc oxide ˝ C to form develop multilayer comb-like zinc oxide nanostructure. aluminum oxide substrate at 950 nanostructure. The experimental results of Tian reveal that the growth of the ZnO nanostructures is The experimental results of Tian reveal thatmechanism the growth and of the nanostructuresprocess is controlled controlled by the vapor-solid (VS) growth is ZnO a diffusion-limited [10]. by the vapor-solid (VS) growth mechanism and is a diffusion-limited process [10]. To fabricate a one-dimensional nanostructure, Zhang et al. successfully placed bismuth (Bi) in a To fabricate one-dimensional nanostructure, et al. successfully placed bismuth in vacuum chamberafor heating and melting, and castZhang the molten Bi into the alumina template(Bi) using ahigh-pressure vacuum chamber for heating and melting, and thetomolten Bimetal. into the alumina using gas [11,12]. A high temperature wascast used melt the The moltentemplate metal material high-pressure gas [11,12]. A high temperature was used to melt the metal. The molten metal material was cast into the nanotemplate by pressurization to fabricate arrayed nanowires. In the fabrication was cast into the nanotemplate by pressurization to fabricate arrayedtemperature, nanowires. In the fabrication of the nanowires, the control variables were pressure value, heating the properties of of the nanowires, the control variables were pressure value, heating temperature, the properties of the liquid metal, and the applied force. However, the pressure of the high-pressure gas was limited the and theTo applied However,that the pressure of theby high-pressure gasmechanically was limited by liquid a gas metal, compressor. solve force. the problems were caused gas injection, by a gas compressor. To solve the was problems were caused by gaspressure injection,inmechanically powered powered hydraulic equipment used that to apply additional a newly developed hydraulic equipment was used to apply additional pressure in a newly developed high-vacuum high-vacuum die casting technique that combined traditional casting with a nano-technique [13].die casting technique that traditional casting withaluminum a nano-technique [13]. templates. The pore Anodization was combined used to fabricate porous anodic oxide (AAO) Anodization was used to fabricate porous anodic aluminum oxide (AAO) templates. pore size of the template was controlled using various solutions. Accordingly, highly regularlyThe arrayed size of the template was controlled using various solutions. Accordingly, highly regularly arrayed nanopores were formed [14]. The depth of the pores was determined by the anodizing duration. nanopores were formed [14]. The depth of theused pores thethe anodizing duration. Then high-vacuum die casting technique was to was cast determined the materialby into nanopores of the Then high-vacuum die casting technique was used to cast the material into the nanopores of template, and thus to fabricate nanowires of different lengths. The purpose of this work isthe to template, and thus to fabricate of different lengths. The purpose work is to fabricate fabricate ZnO nanowire with ananowires diameter of 80 nm inside the nanopores ofof anthis AAO template and to ZnO with a diameter nm inside the nanopores of an template andproperties to study the studynanowire the relationship between of the80length of the ZnO nanowire andAAO the piezoelectric of relationship between the length of the ZnO nanowire and the piezoelectric properties of ZnO material ZnO material with a one-dimensional nanostructure. The results of this research may improve our with a one-dimensional nanostructure. results of this research may improve our understanding of understanding of ZnO material on the The nanoscale. ZnO material on the nanoscale. 2. Experimental Details 2. Experimental Details In the experiment herein, porous AAO film was used as a template, on which zinc foil was In the experiment herein, porous AAO film was used as a template, on which zinc foil was placed. placed. The template was heated until molten. Normal pressure was applied to cast the molten zinc The template was heated until molten. Normal pressure was applied to cast the molten zinc into into the nanopores of the template. Atmospheric annealing was performed to transform zinc the nanopores of the template. Atmospheric annealing was performed to transform zinc nanowires nanowires into ZnO nanowires. Finally, part of the AAO nanotemplate was removed to expose the into ZnO nanowires. Finally, part of the AAO nanotemplate was removed to expose the nanowires. nanowires. Figure 1 shows a flow chart of the experiment. Figure 1 shows a flow chart of the experiment.

Figure Experimental process. AAO: anodic anodic aluminum Figure 1. 1. Experimental process. AAO: aluminum oxide. oxide.

Masuda et et al. al. discovered developed In 1995, Masuda discovered the self-assembling self-assembling porous alumina film, and developed hexagonally arrayed arrayedporous porous alumina [15]. They a two-step anodization grow hexagonally alumina [15]. They usedused a two-step anodization methodmethod to grow to alumina alumina film. Aluminum foil with a purity of 99.7% was used as the substrate. HClO 4 , C 2 H 5 OH, film. Aluminum foil with a purity of 99.7% was used as the substrate. HClO4 , C2 H5 OH, and HOCH2CH2OC4H9 were mixed uniformly in a ratio of 3:14:3 to form an electrolytic polishing

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3 of 10 solution. With pure aluminum foil as the anode and graphite foil as the cathode, electrolytic HOCH OC were mixed uniformly in a ratio of 3:14:3 to form an electrolytic polishing polishing the was performed, completely flattening, lustering, and smoothing itssolution. surface, 2 CH2of 4 H9foil solution. With pure aluminum foil as the anode and graphite foil as the cathode, electrolytic With pure aluminum foil as the anode and graphite foil as the cathode, electrolytic polishing of the foilM favoring the fabrication of the AAO film with very regularly arrayed nanopores. Thereafter, 0.3 polishing of the foil was performed, completely flattening, lustering, and smoothing its surface, was performed, completely flattening, lustering,anodization. and smoothing surface, favoring oxalic acid solution was used in two-phase Withitsaluminum foil as the the fabrication anode and favoring the fabrication of the AAO film with very regularly arrayed nanopores. Thereafter, 0.3 M ofgraphite the AAO with very regularly arrayed 0.3 M oxalic acid solutionThen, was foilfilm as the cathode, 40 V voltage wasnanopores. applied forThereafter, 1 h to perform phase-1 anodization. oxalic acid solution was used in two-phase anodization. With aluminum foil as the anode and used in two-phase anodization. With aluminum foil as the anode and graphite foil as the cathode, 40 V 6% phosphoric acid solution was added to 2% chromic acid solution, and the resulting solution was graphite foil as the cathode, 40 V voltage was applied for 1 h to perform phase-1 anodization. Then, voltage applied 1 hfilm to perform phosphoric acid solution was used towas remove the for AAO that wasphase-1 grownanodization. in phase-1 atThen, 60 °C.6% Finally, phase-2 anodization was 6% phosphoric acid solution was added to 2% chromic acid solution, and the resulting solution was added to 2% A chromic solution, for and5,the resulting solution wasofused to remove the AAO film that performed. secondacid anodization 6, and 7 h yielded films various thicknesses. As presented used to remove the AAO film that was grown in phase-1 at 60 °C. Finally, phase-2 anodization was was grown 2, in phase-1 at 60 ˝ C. phase-2AAO anodization was performed. A second anodization in Figure the nanopores inFinally, the obtained template were very regularly arrayed with a performed. A second anodization for 5, 6, and 7 h yielded films of various thicknesses. As presented for 5, 6, and 7 h yielded of various thicknesses. complete structure, andfilms all pores had a size of 80 nm.As presented in Figure 2, the nanopores in the in Figure 2, the nanopores in the obtained AAO template were very regularly arrayed with a obtained AAO template were very regularly arrayed with a complete structure, and all pores had a complete structure, and all pores had a size of 80 nm. size of 80 nm.

Figure 2. SEM image of alumina template with pores of size 80 nm. Figure 2. SEM image of alumina template with pores of size 80 nm.

Figure 2. SEM image of alumina template with pores of size 80 nm. The nanopores in the AAO fabricated in the experiment are highly regularly arrayed. The diameter of the pores is 80 nm, and the thickness of the tube wall increases with time. The template The nanopores nanopores in the the AAO AAO fabricated fabricated in in the the experiment experiment are are highly highly regularly regularly arrayed. arrayed. The The The helps in fabricating in nanowires, supporting good regularity. The AAO template and zinc film were diameter of the pores is 80 nm, and the thickness of the tube wall increases with time. The template diameter thedie pores 80 nm, the thickness of the tubemold wall increases with time.toThe templateof placed inofthe castismold, asand presented in Figure 3. The was depressurized a vacuum helps in fabricating nanowires, supporting good regularity. The AAO template and zinc film were helps in fabricating nanowires, supporting good regularity. AAO template and zinc film were 2) was 10–3 torr, and heated at 750 °C for 60 min. Then, a hydraulicThe force (kgf/cm applied to cast the placed in the die cast mold, as presented in Figure 3. The mold was depressurized to a vacuum of placed in the die cast mold, as presented in Figure 3. The mold was depressurized to a vacuum of molten zinc into the nanopores of the AAO template. A period of 2condensation yields zinc –33torr, and heated at 750 °C ´ ˝ 2 10 for 60 min. Then, a hydraulic force (kgf/cm ) was applied to cast the 10nanowires. torr, and heated at 750 C for 60 min. Then, a hydraulic force (kgf/cm ) was applied to cast the molten zincinto into nanopores of AAO the AAO template. A of period of condensation zinc molten zinc thethe nanopores of the template. A period condensation yields zincyields nanowires. nanowires.

Figure 3. Die casting mold. Figure 3. Die casting mold.

Figuremetal 3. Die casting mold.the AAO template be proportional to Let liquid thatenters enters Letthe theapplied applied pressure pressure of the liquid metal that the AAO template be proportional to the the surface tension liquidmetal. metal.The Thesurface surfacetension tensionofofzinc zinc in in the the liquid state at C isis surface tension of of thethe liquid at 600 600 ˝°C Let the applied pressure of the liquid metal that enters the AAO template be proportional to the 787 dyne/cm [16]. A hydraulic force was applied to overcome the surface tension of the liquid; 787 dyne/cm [16]. A hydraulic force was applied to overcome the surface tension of the liquid;the the surface tension of the liquid metal. The surface tension of zinc in the liquid state at 600 °C is pressure nanotubewas wascalculated calculatedusing usingEquation Equation (1), where F denotes pressureofofthe the liquid liquid that that enters aa nanotube (1), where F denotes the 787 dyne/cm [16]. A hydraulic force was applied to overcome the surface tension of the liquid; the the normal force; is the surface ofsample; the sample; γ issurface the surface tension the liquid; is the normal force; A isAthe surface areaarea of the γ is the tension of the of liquid; θ is theθcontact pressure of the liquid that enters a nanotube was calculated using Equation (1), where F denotes the contact angle between the liquid and the solid, and r is the diameter of the microtubules. As measured angle between the liquid and the solid, and r is the diameter of the microtubules. As measured by a normal force; A is the surface area of the sample; γ is the tension of the liquid; θ is the contact ˝ . surface by a contact angle analyzer, theofθAAO of AAO 104.85 The diameter of AAO the AAO 80 nm. contact angle analyzer, the θ waswas 104.85°. The diameter of the tubetube waswas 80 nm. The angle between the liquid and the solid, and r is the diameter of the microtubules. As measured by a The surface area samplewas was3.14 3.14cm cm2.2 .Therefore, Therefore,the the additionally additionally applied applied force of surface area of of thethe sample of zinc zincliquid liquid contact angle analyzer, the θ of AAO was 104.85°. The diameter of the AAO tube was 80 nm. The entering ˆ×10 enteringthe thecritical criticalside sideofofAAO AAOwas was3.16 3.16 1088 dyne. dyne. 2 surface area of the sample was 3.14 cm . Therefore, the additionally applied force of zinc liquid entering the critical side of AAO was 3.16 × 108 dyne.

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P “ ´2γ(cosθ/r) pcosθ{rq P =F{A F/A“= –2γ

(1) (1)

The nanowires,following followingdiedie casting, were placed an atmospheric heat treatment The zinc nanowires, casting, were placed in anin atmospheric heat treatment furnace. ˝ furnace. The temperature of thewas furnace was increased to 300 °C and maintained for 36out h tooxidization carry out The temperature of the furnace increased to 300 C and maintained for 36 h to carry oxidization to completely transform zinc intonanowires zinc oxide[17]. nanowires [17]. part template of the AAO to completely transform the zinc intothe zinc oxide A part of theAAAO was template removed using hydroxide 0.1 M sodium hydroxide (NaOH) solution to expose Since the nanowires. removed was using 0.1 M sodium (NaOH) solution to expose the nanowires. part of the Since partremains, of the template remains,maintain the nanowires maintain and theirthe verticality, and the overall structure template the nanowires their verticality, overall structure of the nanowires of the has nanowires array has to facilitate the subsequent measurement of its array enough strength to enough facilitatestrength the subsequent measurement of its piezoelectric properties. piezoelectric properties. deionized waterultrasonic was used cleaning to perform cleaning to after remove Finally, deionized waterFinally, was used to perform to ultrasonic remove the residues the the residues after the template reaction of theNaOH. AAO template withwas NaOH. Bake drying was performed a reaction of the AAO with Bake drying performed on a heating platform. on This heating platform. This concludes the fabrication of the ZnO nanowire piezoelectric component. concludes the fabrication of the ZnO nanowire piezoelectric component. This Thispaper paperuses usesSEM SEMto toobserve observethe thethickness thicknessof ofthe theAAO AAOtemplate, template,and andan anX-ray X-raydiffractometer diffractometer (XRD) to analyze analyzethe the crystal structure of nanowires. This verifies study verifies that if nanowires are (XRD) to crystal structure of nanowires. This study that if nanowires are completely completely transformed into ZnO nanowires, then the crystal direction of ZnO nanowires of three transformed into ZnO nanowires, then the crystal direction of ZnO nanowires of three lengths can lengths can beFinally, observed. Finally, a conductive atomic force microscope is used scan the be observed. a conductive atomic force microscope (C-AFM) is(C-AFM) used to scan theto nanowires nanowires contact andthe measure the piezoelectric current. The length, width, and of in contact in mode and mode measure piezoelectric current. The length, width, and height of height the AFM the AFM cantilever are50 160µm, μm, 50 4.6 μm,µm, andrespectively. 4.6 μm, respectively. height radius of the cantilever are 160 µm, and The heightThe and radiusand of the probe are probe 14 µm are 7 nm, respectively. and147 μm nm,and respectively. Resultsand andDiscussion Discussion 3.3.Results The surface surface morphology morphology of of nanowires nanowires isis observed observed by by scanning scanning electron electron microscopy microscopy(SEM). (SEM). The Figure 4 shows the SEM images of a nanowire that is cast into an AAO template. Figure 4 shows the SEM images of a nanowire that is cast into an AAO

(a)

(b)

Figure Figure4.4.SEM SEMimages imagesof ofnanowires nanowiresthat thatare arecast castinto intoan anAAO AAOtemplate: template:(a) (a)top topview; view;(b) (b)lateral lateralview. view.

Figure 5a shows the SEM front image of the partly removed AAO template. Figure 5b displays Figure 5a shows the SEM front image of the partly removed AAO template. Figure 5b displays the the SEM image of the sample that is slanted 30°. In the figures, the well distribution of ZnO SEM image of the sample that is slanted 30˝ . In the figures, the well distribution of ZnO nanowires array nanowires array can be observed. The ZnO nanowires are found to be very densely arrayed, and can be observed. The ZnO nanowires are found to be very densely arrayed, and each exhibits good each exhibits good verticality. The SEM images in Figure 5a,b reveal that the diameter of the verticality. The SEM images in Figure 5a,b reveal that the diameter of the prepared ZnO nanowires is prepared ZnO nanowires is 80 nm and these nanowires are primarily well aligned on the bottom of 80 nm and these nanowires are primarily well aligned on the bottom of the AOO template. Figure 5c the AOO template. Figure 5c presents the energy dispersive spectrometer (EDS) analytic chart. The presents the energy dispersive spectrometer (EDS) analytic chart. The area of EDS analysis is indicated area of EDS analysis is indicated by the blue square in Figure 5a. Table 1 presents the constituent by the blue square in Figure 5a. Table 1 presents the constituent elements of the fabricated nanowires. elements of the fabricated nanowires. The table reveals that atmospheric annealing can transform The table reveals that atmospheric annealing can transform zinc into ZnO nanowires. Additionally, zinc into ZnO nanowires. Additionally, to elucidate the crystalline quality of the prepared ZnO to elucidate the crystalline quality of the prepared ZnO nanowires, Figure 6 presents a TEM image nanowires, Figure 6 presents a TEM image thereof. The prepared ZnO nanowires are single-crystal thereof. The prepared ZnO nanowires are single-crystal ZnO with a wurtzite structure and a ZnO with a wurtzite structure and a growth direction. growth direction.

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(c) (c) Figure 5. SEM images of ZnO nanowires: (a) top view; (b) lateral view, and (c) EDS pattern. Figure 5. 5. SEM SEM images images of of ZnO ZnO nanowires: nanowires: (a) (a) top top view; view; (b) (b) lateral lateral view, view, and Figure and (c) (c) EDS EDS pattern. pattern. Table 1. EDS analytic composition of ZnO nanowire. Table 1. Table 1. EDS EDS analytic analytic composition composition of of ZnO ZnO nanowire. nanowire.

Element Element OElement K OZnKLO K Zn LZn L Total TotalTotal

Weight % Weight % Weight 14.44 % 14.44 85.56 14.44 85.56 100.00 85.56 100.00 100.00

Atomic %

Atomic Atomic % % 40.82 40.82 40.8259.18 59.18 100.00 59.18 100.00 100.00

Figure 6. TEM image of prepared ZnO nanowires.

Figure 6. TEM image of prepared ZnO nanowires.

6. TEMtemplates image of prepared ZnO nanowires. As shown in Figure Figure 7, the AAO that were anodized for 5–8 h had thicknesses of 49.4 m, 59.5 m, 68.1 m, and 72.6 m. The thickness of each template was determined by the As shown ininFigure 7,anodization. thethe AAO templates that were anodized for 5–8 h had of of 49.4 µm, shown Figure 7, AAO templates that were anodized for 5–8 thicknesses h had thicknesses of duration of the second Figure 8 plots the relationship between the thickness the 59.5 µm, 68.1 µm, and 72.6 µm. The thickness of each template was determined by the duration of 49.4 m, 59.5 m, 68.1 m, and 72.6 m. The thickness of each template was determined by the template and the anodizing duration. When the anodizing time was 5–7 h, the thickness increased at the anodization. Figure plots the relationship thickness of the template and duration of9–10 the μm/h, secondand anodization. Figure 8 plots the between relationship between the thickness the a second rate of when8the anodizing time was 7–8 h, thethe thickness increased at a rateofof the anodizing duration. the anodizing time was 5–7 h,awas the thickness increased at acould rate of template andbecause the anodizing duration. When anodizing time 5–7 h, the the thickness increased at 4–5 μm/h, whenWhen the thickness of thethe template reached certain value, electrolyte 9–10 µm/h, and when the anodizing time 7–8rate h,was the thickness a rate ofat4–5 µm/h, easily the bottom ofwhen the pores, so thewas growth was reduced. a rate ofreach 9–10 μm/h, and the anodizing time 7–8 h, the increased thickness at increased a rate of because when the thickness the template reached a certain value, the electrolyte easily could reach 4–5 μm/h, because when theofthickness of the template reached a certain value, thecould electrolyte the bottom thebottom pores, of sothe thepores, growth reduced. easily reachofthe sorate the was growth rate was reduced.

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Figure7. 7.SEM SEMimages imagesof ofthe thecross-section cross-sectionof ofalumina aluminatemplates templatesthat thatwere wereanodized anodizedfor for5–8 5–8h: h:(a) (a)55h, h, Figure 7. SEM images of the cross-section of alumina templates that were anodized for 5–8 h: Figure (b) 6 h, (c) 7 h, and (d) 8 h. (b) 66h, h,(c) (c)77h, h,and and(d) (d)88h. h. (b) 75 75

Thickness Thickness ( (m m))

70 70 65 65 60 60 55 55 50 50 45 45 40 40 44

55

66

77

88

Time (h) (h) Time Figure8. 8.Relationship Relationshipbetween betweenthickness thicknessof ofalumina aluminatemplate templateand andanodizing anodizingduration. duration. Figure Figure 8. Relationship between thickness of alumina template and anodizing duration.

Figure 99 presents presents XRD XRD diagrams diagrams that that were were obtained obtained at at 0.02° 0.02° intervals intervals of of diffraction diffraction angle angle 2θ 2θ Figure ˝ intervals of diffraction angle 2θ from Figure 9 presents XRD diagrams that were obtained at 0.02 from 30° to 70°, following the XRD measurement of the ZnO nanowires that were made using the from 30° to 70°, following the XRD measurement of the ZnO nanowires that were made using the ˝ to 70˝ , following the XRD measurement of the ZnO nanowires that were made using the AAO 30 AAO template that that was was anodized anodized for for 5–7 5–7 h. h. When When the the diffraction diffraction angle angle 2θ 2θ was was approximately approximately 34.3°, 34.3°, AAO template ˝ , the ZnO template that was anodized for 5–7 h. When the diffraction angle 2θ was approximately 34.3 the ZnO nanowires of all three lengths yielded a very strong diffraction peak. Analysis of of the XRD the ZnO nanowires of all three lengths yielded a very strong diffraction peak. Analysis of of the XRD nanowires of all three lengths yielded a very strong diffraction peak. Analysis of of the XRD chart chart and a comparison with JCPDS database revealed that the peak value was generated by the chart and a comparison with JCPDS database revealed that the peak value was generated by the and a comparison with JCPDS database revealed that the peak value was generated by the ZnO of ZnO of of the the (002) (002) side. side. Therefore, Therefore, the the ZnO ZnO nanowires nanowires that that were were fabricated fabricated in in this this work work exhibit exhibit ZnO the (002) side. c-axis-oriented Therefore, the ZnO nanowires were fabricated work exhibitto preferentially preferentially c-axis-oriented growth, andthe thethat peak strength tendsin tothis beproportional proportional to thelength lengthof of preferentially growth, and peak strength tends to be the c-axis-oriented growth, and the peak strength tends to be proportional to the length of the nanowire. the nanowire. nanowire. The The chart chart also also reveals reveals that that Zn Zn nanowires nanowires have have undergone undergone heat heat treatment treatment at at aa fixed fixed the The chart alsoof Znhhnanowires haveoxidized undergone at ainto fixed temperature temperature ofreveals 300°C °Cthat for36 36 are completely completely oxidized andheat thustreatment transformed into ZnO nanowires.of temperature 300 for are and thus transformed ZnO nanowires. 300 ˝ C for exhibits 36 h are non-ferroelectric completely oxidized and thus transformed into ZnO nanowires. ZnO exhibits non-ferroelectric piezoelectricity. Therefore, cannot generate piezoelectricity piezoelectricity by by ZnO piezoelectricity. Therefore, itit cannot generate ZnO exhibits non-ferroelectric piezoelectricity. Therefore, it cannot generate piezoelectricity by polarization, and and only only aa structure structure that that grows grows along along the the c-axis c-axis isis piezoelectric. piezoelectric. Therefore, Therefore, polarization, polarization, and only a structure that grows along the c-axis is piezoelectric. Therefore, preferentially preferentially c-axis-oriented ZnO is just highly suitable for fabricating piezoelectric components. preferentially c-axis-oriented ZnO is just highly suitable for fabricating piezoelectric components. c-axis-oriented ZnO is just highly suitable for fabricating piezoelectric components.

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Theta (deg.) (deg.) 22 Theta Figure XRD patterns of ZnO nanowires that were fabricated using alumina template that was Figure 9. 9. XRD XRD patterns of of ZnO ZnO nanowires nanowires that that were were fabricated fabricated using using alumina alumina template template that that was was Figure 9. patterns anodized for 5–7 h. anodized for for 5–7 5–7 h. h. anodized

AFM in in contact modewas wasused usedto measurethe thesurface surface morphology over 5 µm 5 µm areas contact mode mode was used totomeasure measure the surface morphology over μm ˆ μm areas of morphology over 55 μm  55 μm areas of of surfaces nanowires thatwere werefabricated fabricatedby bythe theabove above experimental experimental method. method. As As shown shown in thethe surfaces ofof nanowires that were fabricated by the above experimental method. As in the surfaces of nanowires that Figure 10, 10, the the surface surface morphology morphology of of the the ZnO ZnO nanowires nanowires is is very very densely densely arrayed, arrayed, and and each each ZnO ZnO Figure nanowire nanowire exhibits exhibits good good verticality. verticality. verticality.

Figure 10. 10. 3D 3D diagram of surface morphology morphology of of ZnO ZnO nanowires, nanowires, obtained obtained using using atomic atomic force force Figure 10. Figure 3D diagram diagram of of surface surface morphology of ZnO nanowires, obtained using atomic force microscopy (AFM). microscopy (AFM). microscopy (AFM).

Conductive AFM AFM was was used used to to measure measure the the piezoelectric piezoelectric currents currents of of the the ZnO ZnO nanowires nanowires of of three three Conductive Conductive AFM was used to measure the piezoelectric currents of the ZnO nanowires of three lengths. Through Through the the probe, probe, stress stress was was applied applied to to deform deform the the nanowires. nanowires. The The piezoelectric piezoelectric current current lengths. lengths. Through the probe, stress was applied to deform the nanowires. The piezoelectric current that that was was generated generated on on aa certain certain area area of of each each nanowire nanowire was was measured. measured. When When aa current current was was generated generated that was generated on a certain area of each nanowire was measured. When a current was generated on on aa surface, surface, the the probe probe received received aa signal. signal. An An image image of of the the current current distribution distribution on on the the scanned scanned area area on a surface, the probe received a signal. An image of the current distribution on the scanned area was was obtained. obtained. The The current/voltage current/voltage curve curve (I–V (I–V curve) curve) at at aa particular particular point point was was obtained. obtained. Figure Figure 11a–c 11a–c plot plot was obtained. The current/voltage curve (I–V curve) at a particular point was obtained. Figure 11a–c plot the piezoelectric currents on ZnO nanowires that were fabricated using the alumina template that the piezoelectric currents on ZnO nanowires that were fabricated using the alumina template that the piezoelectric currents on ZnO nanowires that were fabricated using the alumina template that had had been been anodized anodized for for 5–7 5–7 h, h, obtained obtained using using C-AFM. C-AFM. As As shown shown in in the the figures, figures, the the piezoelectric piezoelectric had been anodized for 5–7 h, obtained using C-AFM. As shown in the figures, the piezoelectric current current increased increased with with the the length length of of the the nanowire. nanowire. The The nanowire nanowire with with aa length length of of approximately approximately current increased with the length of the nanowire. The nanowire with a length of approximately 70 µm that 70 μm μm that that was was fabricated fabricated after after anodization anodization for for 77 hh generates generates aa current current of of 69 69 pA. pA. Nanowires Nanowires cause cause aa 70 was fabricated after anodization for 7 h generates a current of 69 pA. Nanowires cause a deflection of a deflection of of aa probe probe that that is is applied applied to to stress stress them them under under scanning scanning process. process. As As the the length length of of the the deflection probe that is applied to stress them under scanning process. As the length of the nanowires increases, nanowires increases, increases, the the probability probability of of aa deflection deflection increases, increases, and and larger larger piezoelectric piezoelectric currents currents are are nanowires the probability of a deflection increases, and larger piezoelectric currents are measured. measured. measured.

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(b) (b)

(a) (a)

(c)(c) Figure Piezoelectric current diagrams ZnO nanowires,fabricated fabricatedusing usingalumina aluminatemplate template that Figure 11.11. Piezoelectric current diagrams ofof ZnO nanowires, fabricated using alumina template that Figure 11. Piezoelectric current diagrams of ZnO nanowires, was anodized for 5–7 h, measured using conductive atomic force microscopy (C-AFM): (a) h, was anodized anodizedfor for5–7 5–7 measured using conductive atomic microscopy (C-AFM): was h, h, measured using conductive atomic forceforce microscopy (C-AFM): (a) 5 h,(a) (b) 56 h, h,h.and (b) (b) 6(c) h,67and (c) (c) 7 h.7 h. and

Figure plotsthethecurrent/voltage current/voltageproperty propertycurve curve ofof ZnO ZnO nanowires nanowires measured measured using a Figure 12 12 plots Figure 12 plotsprobe the current/voltage property curve of Schottky ZnO nanowires measured using a platinum-plated serving as a metal electrode. Since contact exists between ZnO platinum-plated probe serving as a metal electrode. Since Schottky contact exists between ZnO platinum-plated probe serving as a metal electrode. Since Schottky contact exists between ZnO nanowires and metal electrodes,the thecurrent/voltage current/voltagecurve curveofofZnO ZnOnanowires nanowires has has an an asymmetric asymmetric nanowires and metal electrodes, nanowires and metal electrodes, the current/voltage curve of ZnO nanowires has an asymmetric property and exhibits property a diode.Therefore, Therefore,a apiezoelectric piezoelectriccomponent component that that is is fabricated fabricated property and exhibits thethe property ofof a diode. property and generates exhibits the property of a diode. Therefore, a piezoelectric component that is fabricated from ZnO a direct current. from ZnO generates a direct current. from ZnO generates a direct current.

Current (pA) Current (pA)

30 30 25 25 20 20 15 15 10 10 5 5 0 0 -5 -5 -10 -10

-6

-6

-4

-4

-2 -2

0

0

2

2

Voltage (V) Voltage (V)

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4

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Figure 12. Current/voltage properties of ZnO nanowires, measured using a platinum-plated probe Figure 12. properties of of ZnO Figure 12. Current/voltage Current/voltage properties ZnO nanowires, nanowires, measured measured using using aa platinum-plated platinum-plated probe probe that serves as a metal electrode. that that serves serves as as aa metal metal electrode. electrode.

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4. Conclusions In this work, AAO templates were made, the formation of nanopores was controlled, and a high-vacuum die casting technique was used to cast zinc into the nanopores of AAO. Zinc was transformed into ZnO nanowires by atmospheric heat treatment, and the AAO template was then removed to expose the nanowires. Microstructural analysis was carried out and observations made. Finally, the piezoelectric current that was generated by ZnO nanowires was measured using C-AFM. The results of this work are summarized as follows: 1. An aluminum template with a purity of 99.7% was anodized to produce an AAO template. The nanopores in the template were highly regularly arrayed and had a high aspect ratio. Finally, process parameters were optimized to minimize the cost of the required consumable materials. 2. The stress associated with the capillary phenomenon in die casting was calculated to obtain the normal force that was required to cast molten zinc metal into nanopores. The pressure was adjusted by using the controller of the die casting machine. A hydraulic force was used to cast molten zinc into the AAO template. After atmospheric heat treatment, arrayed ZnO nanowires were obtained. 3. The nanowires that were fabricated using the AAO template were very dense, had an aspect ratio of over 600, were well arrayed, and exhibited good verticality. 4. The arrayed ZnO nanowires that were fabricated herein exhibited preferentially c-axis-oriented growth. The (002) peak strength was proportional to the length of the nanowire. 5. As observed from the test of piezoelectric properties, elucidated using C-AFM, longer nanowires generated a greater measured piezoelectric current. Among these ZnO nanowires, those fabricated using the alumina template that had been anodized for 7 h generated the greatest piezoelectric current of 69 pA. Acknowledgments: The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 102-2221-E-027-012. Author Contributions: Chin-Guo Kuo is a consultant to the research and supervised the entire research work. Ho Chang is a main writer of the manuscript and advised the experiments. Jian-Hao Wang performed the experiments and assisted the research. Conflicts of Interest: The authors declare no conflict of interest.

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