1、7,8the researchorientedbeenpreparation of ZNTAs, including chemical vapor deposition (CVD)way to synthesize well aligned ZnO nanostructures with uniformsize and large aspect ratio, when the template is removed, the ar-rays would be polluted to some extent, and the dispersed nanorodsor nanotubes woul
2、d agglomerate together, destroying the intrinsicorderedstructures.Forthehydrothermalmethod,thesubstratehasto be dipped inside the requisite solution for days to form tubularZnO nanostructures and the rate of repetition is too low 20,21.Compared with these preparation methods, electrochemical depo-FT
3、O substrate by one-step method and they proposed that the sur-nanotubeforma-the formationprocesssmall diameter in large scale by one-stepelectrodeposition methodand illuminate the nanotube formation mechanism.In this paper, ZNTAs with average diameter less than 200 nmwere synthesized on FTO substrat
4、e by using a one-step electrode-position method. The growth mechanism of the ZNTAs was prelim-inarily discussed, and it is assumed that the critical pH value ofelectrolyte solution played an important role in determining theformation of the ZNTAs. Furthermore, photoluminescence proper-ties of the ZN
5、TAs were also studied.Corresponding author. Tel./fax: +86 1062334926.Journal of Alloys and Compounds 588 (2014) 217221Contents lists availableanE-mail address: (M. Guo).ment, high reaction temperature and particular substrates.Although the template method is a simple and common adoptedwith H genera
6、ted during the growth process by direct electrode-position method. So, it is still a challenge to prepare ZNATs with13, solgel method 14, template method 15, hydrothermalmethod 16,17 and electrochemical deposition method 18,19.Among these various approaches, CVD needs complicated equip-facecondition
7、ofsubstrateplayedakeyroleinthetion, while Hwang 26 put forward a viewpoint thatof the ZnO nanotubes was governed by the self-etching+0925-8388/$ - see front matter C211 2013 Elsevier B.V. All rights reserved.http:/dx.doi.org/10.1016/j.jallcom.2013.11.055struction of nanostructures with well-ordered
8、alignment and mor-phology, iscritical for scientificand technological applications 12.ZnO nanotube arrays (ZNTAs) which have special tubular structure,porosityandlargespecificsurfacearea,wouldleadtoenhancedandnovel functionality in nano electronics and nano photoelectronicsin the future. Till now, m
9、any technologies have been utilized in24, namely, electrodeposition of ZnO nanorods on conducting sub-strates firstly and then etching the top of nanorods by chemical orelectrochemical approach, were used to synthesize ZNTAs. There-fore,howtofabricatethesingle-crystallineZNTAsbyone-stepelec-trodepos
10、ition approach attracted more attention. Tang et al. 25firstly synthesized ZNATs with larger diameters directly on the1. IntroductionOne-dimensional (1D) ZnO nanostructuremost popular topics in material, chemistrygreat potential applications in piezoelectrics2,3, solar cells 46, field emission911 in
11、 recent years. Though lots ofdirected toward preparing randomlynanowire, nanorod and nanotube, it hashas been one of theand physics due to its1, optoelectronicsand sensing devicesefforts are still1D ZnO such asrealized that the con-sitionmethodiswidelyadoptedbecauseofitslowreactiontemper-ature,theco
12、ntrollabilityofthethicknessandshape,highdepositionrate, low-cost equipment and eco-friendly feature, more impor-tantly, the thin films deposited are crystalline structure, so thereis no need to further calcinate it to be crystallized. In recent years,studies on electrodeposition of ZNTAs have been r
13、eported by sev-eral research groups 2226. Generally, two-step method 22ElectrodepositionPhotoluminescenceLetterOne-step electrodeposition of single-crystaland their optical propertiesH. Lu, F. Zheng, M. Guo, M. ZhangState Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological
14、Engineering,article infoArticle history:Received 18 June 2013Received in revised form 30 October 2013Accepted 9 November 2013Available online 20 November 2013Keywords:ZnO nanotube arraysCrystallineabstractZnO nanotube arrays (ZNTAs)deposition method and characterizedand high resolution transmissionZ
15、nO nanotubes with averagemechanism of the ZNTAs wastion spectroscopy were furtherJournal of Alloysjournal homepage: www.elseZnO nanotube arraysUniversity of Science and Technology Beijing, Beijing 100083, PR Chinawere successfully synthesized on FTO substrate using a one-step electro-by scanning ele
16、ctron microscopy (SEM), X-ray diffraction (XRD)electron microscopy (HRTEM). It is shown that the single crystallinediameter smaller than 200 nm grew along 001 direction. The growthalso discussed. Room temperature photoluminescence and UVvis absorp-carried out on ZNTAs to investigate their optical pr
17、operties.C211 2013 Elsevier B.V. All rights reserved.at ScienceDirectd C ExperimentalElectrodeposition of ZNTAs was carried out in a standard three electrodes sys-tem comprising FTO, Pt wire and the saturated calomel electrode (SCE) as working,counter and reference electrodes, respectively. The elec
18、trolyte was 0.005 mol dmC03ZnCl2aqueous solution bubbling with saturated oxygen, and 0.1 mol dmC03KClaqueous solution was introduced as the supporting solution insuring the goodconductivity of the electrolyte. The electrolyte pH value was controlled at 3.70 ad-justed by 0.01 mol dmC03hydrochloride a
19、cid and measured by PHS-25C of ShanghaiKangyi. The electrodeposition potential was controlled at C01.3 V (vs. SCE) for 10 sand C01.0 V (vs. SCE) for 1800 s using CHI 660C potentiostat. The reaction tempera-ture was kept at 80 C176C using a water bath provided with a thermostat.The morphology of the
20、ZNTAs film was characterized by scanning electronmicroscopy (SEM) (Zeiss supra 55) operated at 10 kV. X-ray diffraction (XRD) anal-ysis was performed with a super power X-ray diffractormeter using Cu Ka radia-tion. The high-resolution transmission electron microscopy (HRTEM) and theselected area ele
21、ctron diffraction (SAED) patterns were recorded by Tecnai F20operated at 200 kV. The Ultraviolet and Visible (UVVis) absorption spectroscopywas measured by UVVIS-NIR recording spectrophotometer (Shimadzu, UV-3100)using BaSO4as background and creating the baseline from 240 to 800 nm. The Pho-tolumine
22、scence (PL) spectrum was excited by a HeCd laser (IK Series) with a wave-length of 325 nm at room temperature.3. Results and discussionIn general, ZNTAs on conducting substrates were prepared by atwo-step electrodeposition/chemical reaction approach. In the firststep, ZnO nanorod arrays were usually
23、 obtained according to thefollowing equations:O22H2O4eC0! 4OHC01Zn2xOHC0$ ZnOHx2C0x2ZnOHx2C0x$ ZnOH2OxC02OHC03The total equation can be written as follows:Zn20:5O22eC0! ZnO 4In the second step, as an amphoteric oxide, the obtained ZnO nano-rods whose top surface (001) crystal face) was more unstable
24、 thanother surfaces, could be converted to ZNTAs by selectively beingeroded in acid/alkali solution or salt solution of chloride by formingwater-soluble complexes.Based on the above analysis, it is suggested that we could di-rectly prepare ZNTAs on conducting substrates by one-step elec-trodepositio
25、n method only adjusting the electrolyte pH valuelower than 7, which means keeping the two competitive reactions,namely, ZnO crystal growth and selective self-etching by H+in bal-ance during the electrochemical deposition process.Fig. 1(a) gave the SEM images of as-prepared ZNTAs on FTO sub-strate. I
26、t can be seen that well aligned ZNTAs with uniform size andhexagonal appearance were successfully synthesized in large scaleby using a one-step electrodeposition method, and the averagethickness of the nanotube wall was about 50 nm. Structural prop-erties of single ZnO nanotube were further studied
27、by HRTEM asshown in Fig. 1(b). Both HRTEM and SAED results showed thatthe nanotube was a single crystal. The spacing of the lattice fringeswas about 0.26 nm, which could be indexed as the (002) plane ofwurtzite structure ZnO, suggesting that the growth direction of theZnO nanotube was along 002. Thi
28、s may be ascribed to that H+with critical concentration existed in electrolyte solutions couldpreferentially etch the metastable polar crystal plane (002) ofwurtzite ZnO than other non-polar facets, which can result in theformation of tubular structure and growing preferentially along002 direction.
29、The diameter distribution of the ZNTAs grownon substrate was illustrated in Fig. 1(c). From Fig. 1(c), it can be218 H. Lu et al./Journal of Alloys and Compounds 588 (2014) 217221Fig. 1. (a) SEM images of ZNTAs, (b) HRTEM image of an individual ZnO nanotube. The insetas-prepared ZNTAs, (d) XRD patter
30、ns of ZNTAs on FTO substrate (JCPDS 01-079-0205, rof image (b) is SAED pattern of a single ZnO nanotube. (c) Diameter distribution ofpeaks of FTO substrate).clearly seen that the nanotubes prepared by one-step electrode-position had not only a smaller average diameter (about 188 nm)compared with tho
31、se synthesized by two-step electrodeposition/chemical method (about 500 nm) 22,23, but also a narrower sizedistribution (80% of them were between 150 and 250 nm). TheXRD pattern of as-prepared sample was shown in Fig. 1(d) withall diffraction peaks well indexed to the standard diffraction pat-tern o
32、f hexagonal phase ZnO (JCPDS 01-079-0205), indicating awurtzite structure. In comparison with the standard XRD patternof ZnO, much higher intensity of the (002) diffraction peak indi-cated that the formed ZNTAs were oriented perpendicularly tothe substrate, which can also be confirmed by Fig. 1(a).F
33、or studying the formation mechanism of the ZNTAs, the con-trolled experiments were conducted by varying electrodepositiontime. Fig. 2 gave the SEM images of as-prepared ZnO nanostruc-tures electrodeposited on FTO substrates for different time. It canbe clearly seen that only ZnO nanorod arrays could
34、 be synthesizedon FTO substrate when the electrodeposition time was controlledshorter than 300 s as shown in Fig. 2(a). With the electrodepositiontime increasing from 600 to 1200 s, the shallow pits formed at thecenter part of the nanorods tip turned to deeper ones (Fig. 2(b),and finally well crysta
35、lline ZNTAs were obtained (Fig. 2(c). Furtherincreasing the deposition time to 1800 s, not only the center partsbut the nanotubewalls began to be dissolved, leading to the forma-tion of some damaged ZnO nanotubes as shown in Fig. 2(d). Theaverage diameter and growth density of as-prepared ZnO nano-r
36、od/nanotube arrays were summed up in Table 1. It can be clearlyrod arrays with different diameters. With the deposition proceed-ing, the concentration of Zn2+decreased more seriously than thatof H+in the electrolyte solution. The growth of ZnO nanorodsreached a certain equilibrium and the obtained Z
37、nO nanorodsseemed to have specific average diameter and length under thispreparing condition. So, the rate of dissolution was faster thanthe rate of formation, and the selective self-etching of the formednanorods became the main process. Therefore, the ZNTAs were fi-nally obtained. The previously fo
38、rmed ZnO nanotubes with smallersize would be easier to dissolve in the acid electrolyte solution dueto its high specific surface energy along with electrodepositiontime increasing, meanwhile, the formed ZnO nanotubes with largersize would be apt to grow and remain on the FTO substrate, finally,leadi
39、ng to relatively lower density and larger average diameter ofas-prepared ZnO nanorod/tube arrays.According to the above experimental results, the formationmechanism of aligned ZNTAs may be explained as follows: beforeTable 1The influence of the electrodeposition time on growth density and average di
40、ameterof ZnO nanorod/nanotube arrays.Deposition time (s) Density (rods or tubes/cm2) Average diameter (nm)300 3.52C2109149 10600 3.29C2109159 101200 2.78C2109177 101800 1.78C2109213 10H. Lu et al./Journal of Alloys and Compounds 588 (2014) 217221 219seen that with the electrodeposition time increasi
41、ng from 300 to1800 s, the average diameter increased gradually from 149 10to 213 10 nm whereas the growth density decreased from3.52C2109to 1.78C2109rods or tubes/cm2correspondingly. Thisphenomenon may be explained as follows: in the earlier electrode-position stage, the ZnO seeds with different siz
42、es were firstlydeposited on FTO substrates owing to the ZnOH2C0xxgrowth unitsconglomerated to some extent. Then, the nanorods would prefergrowing from these seeds, resulting in the formation of ZnO nano-Fig. 2. SEM images of ZnO nanorod/nanotube arrays electrodeposited on FTO substratesthe electrode
43、position process, oxygen was bubbling into the pre-cursor solution and adsorbed on the surface of FTO substrate (usedas working electrode). Once the negative potential was applied onthe electrode, the double electrode layer could be formed at theelectrolyte/FTO substrate interface. The reduction of
44、adsorbed oxy-gen on the surface of FTO substrates would take place and lead tothe formation of hydroxide ions (reaction (1). With the presenceof electrostatic field, the hydroxide ions would have an in situ reac-tion with the zinc ions on the substrate surface to form the growthfor different time: (
45、a) t = 300 s, (b) t = 600 s, (c) t = 1200 s and (d) t = 1800 s.for2Compounds 588 (2014) 217221units of ZnOH2C0xx(reaction (2). As a consequence, the heteroge-neous nucleation would occur, the units were easily adsorbed atthe active sites of substrate surface and transformed into zinchydroxide under
46、certain degree of supersaturation (the forwardreaction of Eq. (3) will be favored above 34 C176C). It is believed thatthe morphology of nano ZnO was controlled by the rates of growthalong the (001) plane and the degree of aggregation, high ionproduct of hydroxide and zinc ions (C29Kspof zinc hydroxi
47、de) canlead to a morphology of amorphous ZnO. Otherwise, the crystalli-zation of rod-like ZnO would be favored in a neutral electrolytesolution. With the deposition time increasing, the formed ZnOnanorods growing along directions deviating from the axial direc-tion of the substrate would be easily a
48、pt to meet obstructs. That isto say, the growth of ZnO nanorods in any direction other than ver-tical to the FTO substrate could be restrained. Therefore, with theelectrodeposition time prolonging, the formed nanorods could bealigned and stand perpendicularly to the substrate since nanorodswould be
49、getting denser and squeezed among each other as illus-trated in Fig. 2(a). In our experiment, it is worth noting that thepH value of electrolyte solution was controlled at 3.7, so, the rela-tively higher concentration of H+could lead to etch the metastablepolar crystal plane (001) of wurtzite ZnO easier than other non-polar planes. Obviously, two competitive reactions including wellaligned ZnO nanorods growth and selectively self etching of thena
