Alignment-controlled hydrothermal growth of well-aligned ZnOnanorod arrays

Mudan Wang a, Chengcheng Xing a, Ke Cao a, Liang Meng a, Jiabin Liu a,b,n

a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, Chinab College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

Article history:Received 23 October 2013Received in revised form25 February 2014Accepted 27 February 2014Available online 11 March 2014

Keywords:A. OxidesB. Crystal growthC. Electron microscopyD. Microstructure

a b s t r a c t

ZnO nanorod arrays (ZNAs) were prepared via a two-step seeding and solution hydrothermal growthprocess. Effects of preparing parameters such as seed layer, colloid concentration, substrate andprecursor concentration, on the alignment control of ZNAs were systematically investigated.The deviation angle of ZnO nanorods was measured to evaluate the alignment of arrays. Results showthat seed layer not only controls the vertical orientation of ZNAs, but also the compactness of ZNAs.Altering colloid concentration and substrate can influence the microstructure of ZnO seed layer andaffect the ordered alignment of ZNAs. The precursor concentration has an insignificant effect on thealignment of ZNAs but has great impact on the morphology of ZNAs. Alignment-controlled and well-aligned ZnO nanorods with different diameter and aspect ratio can be obtained by properly controllingthe preparing parameters. A growth mechanism was proposed for the growth of ZnO nanorods.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Since carbon nanotube became the most famous material ofone-dimensional (1-D) systems [1], the synthesis and character-ization of metal oxide nanostructures in the form of nanorods,nanowires, nanotubes and nanobelts, have been receiving exten-sive attention because of increased importance of 1-D structuresfor the electronic transport and optical excitations [2]. Moreover,1-D nanomaterials are promising and significant building blocksfor the potential application in nanoscale optical, electronic,photoelectrochemical devices, especially in photovoltaic devicesand solar cells [3–6]. For example, the mobility of electrons in 1-Dnanostructures is typically several orders of magnitude [7–12]higher than that in nanoparticle films commonly used in dye-sensitized solar cells (DSSCs). It is ascribed to the two differentcharge carrier diffusion mechanisms in nanoparticle films and 1-Dnanostructure films, respectively. The electron transport acrossparticles will dramatically decrease the electron mobility for thesake of recombination loss at the grain boundaries, where electronscattering and charge trapping usually happen by isolated nano-particles, surface states, or defect states [13,14]. On the other hand,1-D nanostructure provides a large area for dye adsorption, direct

transport pathways for photoexcited electrons to electrodes andefficient scattering centers for enhanced light-harvesting effi-ciency [15,16]. Therefore, 1-D nanostructure largely enhances theelectron transport efficiency and finally improves the electronicproperties.

1-D ZnO nanostructures have been regarded as importantmaterials for application in DSSCs due to their excellent electronicand optical properties. These properties propel ZnO to be pro-duced in a wide variety of nanostructures and to present uniqueproperties for electronics, optics and photocatalysis [17–20]. Theelectron transport is reported to be tens to hundreds of timesfaster in nanorod array electrodes than that in nanocrystallineparticulate electrodes in DSSCs [21–23]. So far, a lot of efforts havebeen devoted to the efficient design of 1-D ZnO nanomaterials in ahighly oriented and ordered ZnO nanorod arrays (ZNAs) for thedevelopment of novel devices [24–28]. Greene et al. [29] synthe-sized dense ZNAs with excellent vertical alignment and obtainedperfect performance when applied them to photovoltaic (PV)devices [30]. Law [31] used ZNAs to build DSSCs with conversionefficiencies up to 1.5%. The results indicate that vertical-aligned1-D nanostructures, arrays of nanowires/nanorods or nanotubes,offer significant opportunities to improve the efficiency of solarcells [32]. It means that controlling the alignment of arrays playsan important role in enhancing the electronic and optical proper-ties of 1-D materials. Currently, the methods to prepare excellentarrays of vertically aligned 1-D nanostructures and to control thealignment of arrays are expected.

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Journal of Physics and Chemistry of Solids

http://dx.doi.org/10.1016/j.jpcs.2014.02.0110022-3697 & 2014 Elsevier Ltd. All rights reserved.

n Corresponding author at: Department of Materials Science and Engineering,Zhejiang University, Hangzhou 310027, China. Tel.: þ86 571 87951027;fax: þ86 571 87952290.

E-mail address: liujiabin@zju.edu.cn (J. Liu).

Journal of Physics and Chemistry of Solids 75 (2014) 808–817

To date, a large number of approaches, including vapor-liquid-solid growth (VLS) [33], chemical vapor deposition (CVD) [34,35]and electrochemical deposition (ED) [36], have been used for thesynthesis of well-aligned ZNAs. But these processes requiresophisticated equipment and rigorous conditions such as relativelyhigh temperature, single-crystalline substrate and a chemicalvapour transport and condensation system, and such require-ments will limit the application of this technique in industry.Compared to the VLS, CVD and ED methods, the techniquehydrothermal method, based on low temperature and solutionphase, is more convenient and economic for large-scale prepara-tion of well-ordered ZNAs. Since Greene et al. [29] reported a two-step seeding and solution hydrothermal method, large amounts ofresearches [37–43] such as density control [44,45], and morphol-ogy control [46–48] have been made on the synthesis of verticaland well-aligned ZNAs by the method. Further researches werecarried out in hope to control the alignment of ZNAs. Ladanov [49]and Peiris [50] studied the effects of seeding layer, including thethickness, surface roughness and crystalline orientation on thewell-aligned growth and properties of ZnO nanorods/nanowires.They found that the final synthesized nanorod/nanowires weredependent on the quality of the seeding layer, and carefulpreparation of the seeding layer allowed for good control overthe properties. Lee et al. [51] discussed this issue and demon-strated that the alignment degree of ZNAs could be controlled bymonitoring the ambient relative humidity (RH) level during theseeding step. Close-packed ZnO nanorod arrays with [0 0 1] axisperpendicular to the substrate were obtained only when seeded atRH 420% at room temperature. An optimal seeding RH for thesynthesis of ordered ZnO nanorod arrays was 30–40%. Guo et al.[52] studied the effect of preparing conditions such as pre-treatment of substrate, growth temperature and the concentrationof precursor on the morphology and the alignment ordering ofZnO nanorod arrays. The influence of preparing conditions on roddiameter, distribution and aspect ratio of arrays was demonstratedin detail, while little information was focused on the research withrespect to the arrays alignment.

Although some achievements have been made on the align-ment control of ZNAs from the studies mentioned above, there arestill two main problems existed. First, a variety of preparingparameters such as substrate, seed layer, growth temperature,reaction time, seed concentration, precursor concentration and soon will interact with each other in hydrothermal process andproduce its perspective influence on the synthesis and alignmentcontrol of ZNAs. Previous researches basically focused merely onone or two aspects and detailed systematical studies on theseissues still highly need to be carried out. Second, as for alignmentcontrol, it still lacks a specific and measurable characterizationmethod to illustrate the degree of alignment. Existing researchesmainly used the results from scanning electron microscopy (SEM)and X-ray diffraction (XRD) to indicate the alignment degree ofarrays. In this paper, the effects of preparing parameters, seedlayer, colloid concentration, substrate and precursor concentration

on the alignment of ZNAs were systematically studied. A detailedand measurable approach was applied to characterize the align-ment degree of ZNAs. The alignment of the as-prepared ZNAs canbe effectively controlled under suitable preparation conditions andit can be clearly displayed from the distribution of deviation angle.

2. Experimental

2.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2 �6H2O, AR), hexamethyle-netetramine (HMTA) (C6H12N4, AR), zinc acetate dehydrate(Zn(CHCOO)2 �2H2O, AR), ethanolamine (MEA) (NH2CH2CH2OH,AR) and 2-methoxyethanol (CH3OCH2CH2OH, AR) were used asstarting materials. Fluorine tin oxide (FTO) glass and Si(1 0 0)flakes were used as substrates and ultrasonically cleaned inacetone, ethanol and de-ionized water for 15 min each,respectively.

2.2. Preparation of seed layer

A seed solution was prepared by sol–gel method. Zinc acetatedehydrate and MEA of equivalent molar were dissolved in2-methoxyethanol at ambient temperature. The concentration ofZn2þ was controlled by changing the molar ratio of zinc acetateand MEA to 2-methoxyethanol. Then the mixture was magneticallystirred at 60 1C for 30 min to obtain a transparent and homoge-neous solution. The substrates were dipped into the seed solutionwhich is also called “colloid” in some parts of this paper and pulledup at a rate of 200 mmmin�1, and then the coated substrates weredried at 80 1C in an oven. The procedure above was repeated threetimes. Finally, the substrates were heat-treated at 300 1C for 10 minand then heated to 550 1C at a rate of 2 1C min�1 for 1 h to obtainuniform and dense ZnO seed layers on the substrates.

2.3. Growth of ZNAs

A precursor solution for the growth of ZNAs was prepared bydissolving zinc nitrate hydrate (Zn(NO3)2 �6H2O) and HMTA(C6H12N4) in de-ionized water. The molar ratio of Zn2þ to HMTAwas 1:1. Then the precursor solution was put into a Teflon-linedautoclave of 25 mL capacity and the seed-coated substrates wereput face down (FTO substrate: side with FTO; Si substrate:polished side) in the solution, and the autoclave was sealed andheated at 90 1C for 6 h. After the heating treatment, the autoclavewas cooled to room temperature, and the substrates wereremoved from solution, rinsed with de-ionized water 2–3 times,and then dried in air.

2.4. Characterization

All observation and characterization were performed andanalyzed on the face side. The crystallinity and structure of the

Fig. 1. Definition of the deviation angle of arrays.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817 809

seed layer and the nanorod arrays were analyzed by XRD (PANa-lytical X'Pert PRO with CuK radiation). Atomic force microscope(AFM, Bruker dimension edge) was used to observe the surfacemorphology of the seed layers on different substrates. SEM(Hitachi SU-70) and high resolution transmission electron micro-scope (HRTEM, JEM-2100) were used to examine the morphologyand atomic arrangement of the arrays. In addition, a way to definethe deviation angle of ZnO nanorods from vertical was shown inFig. 1 and relevant computer software was applied to measure thedeviation angle (θ) of arrays. To reflect the large-area alignment ofZNAs as much as possible and as true as possible, the test samplesfor observing cross-view SEM images were cut from four differentdirections around the sample. And then the cross-view SEMimages of a sample were observed along the four cross sectionsrespectively. So that the resulting deviation angle distribution ofthe arrays obtained in a round are guaranteed to be real andillustrate the level and variation tendency of the arrays alignment.Each data point was obtained from three different areas and eachtime two hundred nanorods were measured along one crosssection.

The specific procedures for choosing two hundred nanorodsand measuring deviation angle are described as follow: first, weimport a cross-section SEM image into an image processing soft-ware. A line perpendicular to the substrate is plotted in the image.A straight line is plotted along the axis of an individual ZnOnanorod and intersects the line perpendicular to substrate. Anangle between the two lines is formed and the value is directlyobtained from the computer software. The rest nanorods aremeasured by the same method and the deviation angles of twohundred nanorods from one cross section are obtained. Finally,we consolidate all the measuring result from three differentareas, each with four cross sections, namely two thousand andfour hundred nanorods, and obtain the deviation angle distribu-tion of two thousand and four hundred ZnO nanorods from adata point.

3. Results and discussion

3.1. Structure and morphology

Fig. 2a and b shows the SEM images of the as-synthesized productgrown on seed-coated FTO substrates. The low-magnificationimage displays large-area uniformity and high compactness of rodarrays with diameters ranging from 50 nm to 80 nm. The high-magnification tilt-view image displays a large scale of verticallyaligned nanorod arrays with maximum length to 2 μm. The phaseidentity of the as-synthesized products on FTO substrate wasdetermined by XRD (inset in Fig. 2a). All diffraction peaks presentedcan be indexed to the hexagonal wurtzite structure of ZnO accordingto JCPDS#36-1451. In addition, the diffraction lines of SnO2 from FTOsubstrate were also distinctly observed. In comparison with standardpowder diffraction data (JCPDS#36-1451), the intensity of (0 0 2)peak is extremely stronger, which implies that the as-synthesizedZnO nanorods preferentially oriented in the direction of c-axis.

Representative TEM image of single-crystal nanorod orientedalong the ½2110� zone axis was indexed to the hexagonal structureof ZnO by the corresponding select area electron diffraction (SAED)pattern (Fig. 2c). The atomic arrangements of the ZnO nanorodswere clearly observed under HRTEM (Fig. 2d). The averagedistance between the parallel lattices was measured to be0.52 nm, corresponding to the interval of (0 0 0 1) planes. Thisresult demonstrates that the ZnO nanorods grow along the c-axis[0 0 0 1], which accords well with XRD analysis and the previousinvestigation [53].

For further investigation of the interface science between ZnOseed and ZnO rod, the cross-section transmission electron micro-scope (XTEM) was carried out and shown in Fig. 3. The sequentlayers of glass, SnO2, ZnO seed and ZnO nanorods are clearlyobserved in Fig. 3a. SAED pattern is taken at the interface betweenZnO seed and ZnO nanorod (white rectangle in Fig. 3a) and shownin Fig. 3b. The beam incident is parallel with the ½2110�zone axis

Fig. 2. Vertically aligned ZnO nanorod arrays grown on seed-coated FTO substrates with colloid concentration 0.75 M and precursor concentration 0.025 M. (a) Top-viewSEM image and the XRD pattern as inset; (b) tilt-view SEM image; (c) TEM image and SAED pattern as inset and (d) HRTEM image corresponding to the red rectangle in (c).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817810

of the ZnO nanorod. An extra diffraction spot appears close to theð0111Þ spot in the SAED pattern and is indentified to be ð0110Þ.When using the ð0111Þ and ð0110Þ spots to obtain dark-field TEMimage, the ZnO seed and ZnO nanorod show bright contrast whilethe rest shows dark contrast. It indicates that the ð0110Þ spotshould be from the ZnO seed. This result suggests that the ZnOseed have no special orientation relationship with the ZnO rod, butsome crystal planes keep nearly parallel with each other, such asthe ð0110Þ seed nearly parallel with ð0111Þ rod. The correspond-ing HRTEM image is shown in Fig. 3c, which agrees well with thededuction from Fig.3b.

3.2. Effects of preparing conditions on the alignment of ZNAs

3.2.1. The effect of seed layer on the vertical alignment of ZNAsThe structure and morphology of seed layer were characterized

by XRD and SEM. As shown in Fig. 4, dense and uniform nanocrystalseeds were coated onto the substrate. The seeds have good crystal-linity and generally display hexagonal morphology. The size of theseed particles ranges from 50 nm to 100 nm. The XRD resultdemonstrates that the structure of seed layer is close-packedhexagonal wurtzite ZnO. In addition, the high relative intensity of(002) peak reveals that the seeds have strong c-axis texture.

To illustrate the effect of seed layer on the vertical alignment ofZNAs, a comparison experiment was designed by using ZnO-seed-coated FTO substrates and bare FTO substrates to grow ZnOnanorods. It can be seen that the nanorods grown on bare FTOsubstrates (Fig. 5a) rest dispersedly on the substrate and shownearly random orientation. However, the nanorods grown onseed-coated FTO substrate show better alignment, orienting in adirection normal to the substrate, and have higher density(Fig. 5b). The result indicates that the seed layer can facilitatethe ordered and vertical growth of ZnO nanorods on substrates.The reason can be attributed to lower nucleation energy barrier

and easier heterogeneous nucleation process due to the matchinglattice structure of ZnO seed layer. In addition, uniform surfaceroughness of ZnO seed layer can provide more nucleation sites andrestrict the disorder migration of nucleation sites [54].

3.2.2. The effect of colloid concentration on the alignment of ZNAsThe results above demonstrate that ZnO seed layer plays an

important role for the vertical alignment of ZNAs. The structureand quality of a seed layer can be influenced by many parametersin the process of coating a seed layer, and colloid concentration isthe most important one. Therefore, it is significant to adjust theconcentration of seed solution to clarify the effect of colloidconcentration on the alignment of ZNAs. From the SEM imagesand deviation angle distributions of the ZNAs (Fig. 6), it can beclearly seen that the morphology and alignment degree of ZNAschange greatly with colloid concentration. When the colloidconcentrations are 0.05 M and 0.5 M, the substrates are coveredwith ZnO nanoparticles and display high roughness (insets inFig. 6a and c). The density, number of ZnO seed particles per unitarea, of seed layer increases with the colloid concentrationincreasing from 0.05 M to 0.5 M. As a result, the compactness ofthe as-grown ZNAs increases correspondingly and the alignmentof the as-prepared ZNAs alters for the better with the increase ofcolloid concentration. When the colloid concentration is furtherincreased to 1 M, the substrate is completely covered by a flat,uniform and dense ZnO seed layer. Many seeds display typicalhexagonal rod-like shape. The reason may be for that the furtherincrease in colloid concentration leads to the formation of anexcess of nucleation sites, and then the continuous nucleationprocess terminate and the major process is the coalescence of theadjacent seeds and the crystal growth along c-axis direction.Finally, many seeds have grown into short hexagonal rods in

Fig. 3. (a) XTEM bright-field image of ZnO ZNAs, (b) XTEM dark-field image and corresponding SAED pattern, and (c) HRTEM image of the ZnO rod and seed of the alignedZnO nanorod arrays grown on seed-coated FTO substrates with colloid concentration 0.75 M and precursor concentration 0.025 M.

Fig. 4. (a) SEM image and (b) XRD pattern of seed layer with colloid concentration 0.75 M.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817 811

advance of the hydrothermal growth. As a consequence, the as-grown ZnO nanorods on the seed layer display a broad diameterdistribution from 40 nm to 120 nm (Fig. 6e).

The deviation angle distributions of ZNAs obtained from cross-view SEM images are shown in Fig. 7. The deviation angledistributions of the arrays mostly focus on the range 01–21.

Fig. 5. SEM images of ZnO nanorods grown on two different substrates with precursor concentration 0.75 M. (a) Bare FTO and (b) seed-coated FTO, insert is thecorresponding cross-section SEM images.

Fig. 6. SEM images of ZnO seed layers (insets) and ZnO nanorod arrays grown on the seed layers with precursor concentration 0.025 M and different colloid concentration. (aand b) 0.05 M; (c and d) 0.5 M and (e and f) 1 M.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817812

In addition, the proportion of nanorods distributed in the range(01–21) ascends with the increase of colloid concentration from0.05 M to 1 M and the distribution range of deviation anglenarrows down accordingly. It implies that adjusting the colloidconcentration of seeding can be an effective way to control thealignment of arrays. The weighted mean (x) and standard devia-tion value (S) of the θ are calculated to simply evaluate theorientation and distribution of ZNAs. The weighted mean andstandard deviation are calculated by the following equations:

x ¼ ∑f Xn

ð1Þ

s¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi∑f X2�ð∑f XÞ2=n

n�1

sð2Þ

where f¼ frequency; X¼class mid-value; n¼∑f .The calculated deviation angle of ZNAs grown on seed layers

with colloid concentrations 0.05 M, 0.5 M and 1 M are12.11710.11, 5.6176.41, 4.7175.41, respectively. The weightedmean value comes closer to 01 and the better orientation theZNAs has. Moreover, the larger the standard deviation is and thebroader the distribution range of deviation angle is. The calcula-tion results are well consistent with corresponding distributiondiagrams.

3.2.3. The effect of substrate on the alignment of ZNAsAlthough the seeds are coated on FTO substrate by sol–gel

method, it still needs to be clarified whether well-oriented ZNAscan be obtained on arbitrary substrates covered with ZnO seedlayers or different substrates result in extremely different align-ment level of ZNAs. Hence, it is required to choose differentsubstrates for the coating of seed layers, with the goal to researchthe influence on alignment of ZNAs. As shown in Fig. 8, better andlonger ZnO nanorods were obtained on seed-coated FTO substrate

(�50 nm in width and �1 μm in length) than those on the seed-coated Si substrate (�50 nm in width and �500 nm in length). Inaddition, the deviation angle distribution on the seed-coatedFTO substrate is much narrower and denser than that on theseed-coated Si substrate. The percentage of ZnO nanorods on seed-coated FTO substrate distributed in the range 01–21 is approxi-mately twice as much as that on seed-coated Si substrate(Fig. 8e and f). The calculated deviation angle θ on average ofZNAs on FTO and Si substrates are 3.4173.41 and 10.9179.61,respectively. In short, FTO substrate is more suitable for hydro-thermal growth of well-aligned ZNAs than Si substrate.

To figure out the difference between the two substrates, afurther research on the two seed-coated substrates was carried outunder AFM. The insets in Fig. 8 give the AFM images of the ZnOseeds on the two different substrates, respectively. The meanroughness, characterized and revaluated by AFM, of ZnO-seed-coated FTO and ZnO-seed-coated Si substrates are 8.7 nm and5.1 nm, respectively. The seed layer on FTO substrate has higherroughness and consists of seed particles with smaller size. Thismay be responsible for the difference of the ZnO nanorods grownon the two different substrates. Higher surface roughness meanshigher specific surface area and therefore offers more nucleus sitesand brings easier growth for ZNAs. Besides, the interactive effectamong ZnO nanorods is inevitable and intensifies with theincrease of arrays density. To a large extent, high density caninhibit the free growth of nanorods in any direction. As a result,higher density and better verticality of ZNAs can be obtained onZnO-seed coated FTO substrate.

3.2.4. The effect of precursor concentration on the alignment of ZNAsFig. 9 shows the morphology and the orientation degree of ZnO

nanorods grown in different initial precursor concentrations,respectively. Besides, the deviation angle distributions of thearrays further illustrate the alignment degree of the arrays in

Fig. 7. The deviation angle distribution of ZnO nanorods grown on seed layers with a precursor concentration of 0.025 M and different colloid concentration. (a) 0.05 M;(b) 0.5 M and (c) 1 M, and (d) percentage of vertical-aligned (θ¼0o) nanorods varied with colloid concentration.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817 813

detail (Fig. 10). Combining Figs. 9 and 10, the result reveals thatprecursor concentration has slight effect on the vertical alignmentof the ZNAs grown on substrates. When the concentration ofprecursor solution is 0.005 M, the nanorods grow in any directionswith large variation in orientation (Figs. 9 and 10a), which is inaccordance with the deviation angle θ of 11.7179.01. When theprecursor concentration increases to 0.025 M, the orientation ofZnO nanorods is greatly improved, with the rod axis approxi-mately parallel to each other and deviation angle θ of 2.8172.61.Besides, the deviation angle distributes in a smaller range and thepercentage of vertical aligned nanorods is more than twice asmuch as that in 0.005 M solution system (Fig. 10a and b). Dense,uniform and vertical-aligned ZNAs are obtained on ZnO seed-coated substrates. When the concentration is further increasedfrom 0.05 M to 0.1 M, the vertical alignment of ZNAs hardlychanges, and the deviation angle distribution nearly keeps thesame (Fig. 10c and d). The calculated deviation angle θ are2.4172.21 and 2.4172.21, respectively. Besides, the percentageof vertical-aligned nanorods almost stays steady at 65% (Fig. 10e).But the diameter of nanorods increases obviously. It can be clearly

seen that the rods have larger diameter and the arrays are close-packed with each other (inset in Fig. 9d).

The possible reason is explained as follows. The increase ofprecursor concentration will lead to the formation of morenucleation sites on the substrate surface, as well as, the increaseof ZNAs density and the enhancement of ZNAs verticality due tothe interaction among nanorods. When the precursor concentra-tion is further increased, the nucleation process will terminate andthe density of ZNAs remains approximately steady [45]. Becausethe density is decided by the number of nuclei formed at thebeginning of growth. The later arrival of more ions will facilitatethe growth of nuclei with size bigger than the critical size,according to the critical nucleation size theory [45]. Therefore,when the precursor concentration is low, the increase of precursorconcentration can improve the alignment of ZNAs to a largeextent, however, when the precursor concentration is highenough, further increase of precursor concentration has noobvious help to the alignment of arrays. But it will make aremarkable change to the morphology and the aspect ratio ofZnO nanorod. As can be seen in Fig. 9, with the increase of

Fig. 8. SEM images and AFM images (insets) and deviation angle distribution of ZnO nanorod arrays grown on two different substrates with colloid concentration 0.75 M andprecursor concentration 0.025 M. (a, b and e) seed-coated FTO substrate and (c, d and f) seed-coated Si substrate.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817814

precursor concentration from 0.025 M to 0.1 M, the aspect ratio ofZnO nanorods decreases and the average value calculated are 19.8,18 and 14.5, respectively.

3.3. Growth mechanism

In our reaction system, Zn2þ and OH� were provided by thehydrolysis of Zn(NO3)2 and HMTA, respectively. The chemicalreactions occurred to produce ZnO nanorods are formulated asfollows [55]:

ðCH2Þ6N4þH2O-4NH3ðgÞ þ6HCHOðgÞ ð3Þ

NH3þH2O-NHþ4 þOH� ð4Þ

Zn(OH)2 complexes are formed via the chemical reaction betweenZn2þ and OH� , which acts as growth unit of ZnO nanostructures.When the concentrations of Zn2þ and OH� ions exceed a criticalvalue, the precipitation of ZnO nuclei begins. The chemical reactionsof Zn(OH)2 decomposition and ZnO nucleation are described asfollows [56]:

Zn2þ þ2OH�-ZnðOHÞ2 ð5Þ

ZnðOHÞ2-ZnOþH2O ð6ÞThe shape of ZnO nanostructures depends on the growth habit

of hexagonal wurzite structure of ZnO. ZnO is a polar crystal andconsists of polar and non-polar faces. In polar ZnO crystals, thezinc and oxygen atoms are arranged alternately along the c-axis.The top surfaces are Zn-terminated (0001) and are catalyticallyactive, while the bottom surfaces are O-terminated ð0001Þandchemically inert. Generally, the final morphology of ZnO crystals isdetermined by the slowest growing planes. In hydrothermalpreparation of ZnO system, the growth velocity (v) of differentindex planes is described as follow [56]: vð0001Þ4vð101 1Þ4vð1010Þ4vð1011Þ4vð0001Þ. Thus the rod-type morphology is

frequently obtained. Furthermore, the rapid growth rate along[0 0 0 1] direction facilitates the c-axis alignment of ZNAs.In addition, the alignment is also closely related to some otherconditions such as seed layer, colloid concentration, substrate andprecursor concentration.

On the basis of above reaction and analysis, a growth mechan-ism was proposed. Fig. 11 shows a schematic illustration for thegrowth and alignment control of ZNAs. The pre-coated seed layerplays a crucial role for well-aligned ZnO nanorod growth. The seedlayer coated on FTO substrate has a thickness of 60 nm and aroughness of 8.7 nm, according to the data from cross-section SEMand AFM images. The structure of seed layer is close-packedhexagonal wurtzite ZnO and show high relative intensity of(0 0 2) peak, revealing that the seeds have strong c-axis orienta-tion. During the latter solution growth of ZnO nanorod on the seedlayer, zinc and oxygen atoms are arranged alternately on the top ofZnO seed layer and therefore new ZnO crystals nucleate on theZnO seed. With the help of ZnO seed layer as nucleation sites,plenty of ZnO nuclei would form and grow. At the beginning stageof growth, those ZnO nuclei may grow in various directions.However, since many other ZnO nuclei exist around every ZnOnucleus, the growth of the ZnO nucleus whose c-axis is notperpendicular to the ZnO seed layer would be always inhibitedby the surrounding nuclei. Therefore, after a stage similar to thesurvival of the fittest in nature, the survived crystals have theirc-axis nearly perpendicular to the ZnO seed layer and ZnOnanorod arrays form. It is not necessary for the ZnO nanorodgrowing along the normal direction of the ZnO seed. However, theZnO nanorod usually has some planes nearly parallel with the ZnOseed as revealed by XTEM in Fig. 3, which should benefit thenucleation and growth of the ZnO nanorod. The major effect ofZnO seed layer may be that it provides high density of nucleationsites for the ZnO nanorods to grow and compete.

When the colloid concentration is low, the formation velocitiesof the ZnO nanoparticles were too slow to form ZnO-nanoparticle

Fig. 9. SEM images of ZnO nanorods grown with different precursor concentrations and colloid concentration 0.75 M. (a) 0.005 M, (b) 0.025 M, (c) 0.05 M and (d) 0.1 M.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817 815

seed layer on the entire surface of substrate. Therefore, the grownnanorods on uncovered area were randomly distributed instead ofaligned ones on area covered by seed. On the other hand, whether

the microstructure of the substrate surface is flat or not can alsoinfluence the orientation of seed and then the final alignment ofarrays. Different substrate acts on the same seed layer and resultsin different roughness of seed layer. Substrate surface is notsmooth enough at the nanoscale so that not all the (0001) planesof the seed particles are parallel to the substrate. The nanorodsgrown on the nucleation seeds with the (0001) planes deviatingfrom the parallel plane of the substrate will grow along thedirection deviating from the axial direction of the substrate.

In the next solution growth process, the main factor which hasan influence on the growth and alignment of ZNAs is precursorconcentration. Results demonstrate that precursor concentrationhas a little effect on the alignment of ZNAs but has great impact onthe density and aspect ratio of ZNAs. In initial growth stage, theZnO nanoparticles served as seeds and supported the subsequentgrowth of highly oriented ZnO nanorods. Meanwhile, the freshnucleation occurred with the growth of ZnO nanocrystals. Finally,the density of the arrays increases with prolonging reaction timeand increasing precursor concentration. Hence, to a large extent,higher density results in better alignment of ZNAs. When theprecursor concentration increases to an extent, the supersaturated

Fig. 10. The deviation angle distribution of ZnO nanorods grown on seed layers with different precursor concentration and a colloid concentration of 0.75 M. (a) 0.005 M;(b) 0.025 M; (c) 0.05 M and (d) 0.1 M, and (e) percentage of vertical-aligned (θ¼0o) nanorods varied with precursor concentration.

Fig. 11. Schematic illustration of growth mechanism of the ZNAs.

M. Wang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 808–817816

density achieves and the growth of ZnO nanorod is suppressed.Changing the precursor concentration brings about an obviouseffect on the diameter and aspect ratio of ZnO nanorods, especiallyin low zinc concentration system. Low zinc concentration facil-itates the growth of long and fine ZnO nanorods. Generally, thediameter and aspect ratio decrease with the reduction of zincprecursor concentration. The reason can be explained by thedissolution-regrowth and transport limitation in solution system.Because of a much larger contact interface with the precursorsolution, the dissolution of six non-polar ð0110Þ planes proceedsinstead of the high-energy polar (0001) planes. The formed zincions directly transfer to the top of ZnO nanorod, namely, polarZn-terminated (0001) planes with high chemical activity. It leads to adecrease of the diameter and an increase of the length andaspect ratio.

4. Conclusion

In summary, it is demonstrated that the preparing parameters,seed layer, ZnO colloid concentration, substrate and precursorconcentration, have different degrees of influence on the verticalalignment of the ZNAs. Oriented seed layer plays a key role infacilitating the vertical alignment of ZNAs. Adjusting the colloidconcentration of seed layer can be an effective way to control thealignment of arrays. The alignment of ZNAs is improved when thecolloid concentration increases from 0.05 M to 1 M. The deviationangle distribution of arrays narrows down with the increase ofcolloid concentration and the proportion of vertical alignednanorods enlarges correspondingly. The alignment, crystallinityand morphology of ZNAs grown on seed-coated FTO substrate aremuch better than that on seed-coated Si substrate. Precursorconcentration has an insignificant effect on the alignment of ZNAs,but it plays an important role in controlling the morphology ofZnO nanostructure with respect to the shape, diameter and aspectratio. When the precursor concentration is as low as 0.005 M, theincrease of precursor concentration can improve the alignment ofZNAs to a large extent. However, when the precursor concentra-tion is as high as 0.1 M, the alignment of arrays nearly keep steadywith the vertical alignment percentage constant and furtherincrease of precursor concentration can hardly change the align-ment of arrays.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Nos. 21301155 and 50871103).

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