Morphology control of ZnO bilayer structure by low-temperature hydrothermal process Yanyan Lou a,b , Shuai Yuan a, n , Yin Zhao a , Pengfei Hu b , Zhuyi Wang a , Meihong Zhang a , Liyi Shi a a Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, China b Laboratory for Microstructures, Shanghai University, 99 Shangda Road, Shanghai 200444, China article info Article history: Received 7 April 2013 Accepted 17 May 2013 Available online 5 June 2013 Keywords: Semiconductors Thin film Multilayer structure ZnO abstract Morphology control of ZnO bilayer structure films has been obtained via low-temperature hydrothermal process without any surfactant-assistance. ZnO bilayer structures including nanoflower layer on nanorod array structure, nanodendrite layer over nanorod array structure, and double nanorods layers structures were successfully fabricated by controlling the ammonia concentration in the initial hydrothermal solution and the immersing ways. The shape evolution of bilayer structure consisting of nanodendrite layer on the top and nanorod array at the bottom was elucidated by growing–renucleating mechanism. The higher ammonia concentration in the initial hydrothermal solution promotes the secondary nucleation and growth on the surface defects of backbones, resulting in the formation of needle-like nanorods in ZnO nanodendrite–nanorod structure. The ZnO film with nanodendrite–nanorod structure has better optical properties and lesser defects compared to that of nanoflower–nanorod structures. & 2013 Elsevier B.V. All rights reserved. 1. Introduction ZnO is a direct band gap semiconductor with wide energy band gap (3.37 eV), high electron mobility (115–155 cm 2 V −1 s −1 ) and simple fabrication of ZnO nanostructures which make it useful for photoelectron devices such as light emitters [1], transparent conducting oxides (TCOs) [2], dye sensitized solar cells [3] and gas sensors [4]. It is necessary to fabricate ZnO nanostructures and control its size and shape for specific functional purpose. Various ZnO nanostructures such as nanorods [5], nanoflowers [6] and nanosheets [7] have been prepared by hydrothermal processes, allowing for low cost and readying extension to large scale fabrication. Previously, ZnO nanorods array could be obtained by tuning the pH [8], process temperature [9], and concentration of precursors [10,11]. In this work, nanoflower–nanorod, nanodendrite–nanorod and double nanorods layers structures were fabricated by controlling growing condition via low- temperature hydrothermal process. The possible shape evolution mechanism of nanodendrite–nanorod structure was discussed. 2. Experimental details Firstly ZnO seed layer was prepared by dip-coating an FTO substrate in a solution of ehylene glycol monomethyl ether with 1:1 M ratio of 0.375 M zinc acetate and ethanolamine, and annealing at 550 1C for 90 min. The nanostructures were fabri- cated by using hydrothermal method in Teflon-lined autoclaves, where the substrates with the seed layer were immersed into 75 ml aqueous solution of 0.12 M Zn(NO 3 ) 2 Á 6H 2 O and NH 3 Á H 2 O. The autoclaves were sealed and heated at 90 1C for 4 h; then the solution was refreshed and the growth was repeated for another 4 h. Samples A and B were faced upward and immersed slantways in 0.96 M NH 3 Á H 2 O and 1.2 M NH 3 Á H 2 O, respectively; sample C was faced downward and immersed in 1.2 M NH 3 Á H 2 O. The morphologies and crystal structure of different ZnO nanostruc- tures were characterized by a JEOL JSM-6700F and Rigaku D/max 2500 X ray diffractometer with CuKα radiation. The photolumi- nescence spectrum was excited by a 330 nm Horiba HR800 UV laser at room temperature. 3. Results and discussion Fig. 1 shows the top and side-view SEM images of samples A–C. Sample A is the nanoflower–nanorod structure consisting of nanoflower layer on top and nanorod array at bottom. The flower-shaped nanostructure is 10 mm in diameter and is com- posed of many hexagonal plane-tipped nanorods with diameters of 200–300 nm and lengths of 5 mm. The bottom layer of sample A is a nanorods array about 6 mm in thickness. Sample B has a nanodendrite–nanorod structure with nanodendrite layer on top and nanorod array at bottom. The nanodendrite shape is quite Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ -see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.072 n Corresponding author. Tel./fax: +86 21 66134852. E-mail address: s.yuan@shu.edu.cn (S. Yuan). Materials Letters 107 (2013) 126–129 similar to the flower shape of sample A. The branches of nanoden- drite structure are needle-like nanorods with diameters of 100 nm and lengths of 800 nm, while the backbones have 400–500 nm diameters and 5–6 mm lengths. Compared to sample A, the back- bones of nanodendrite structure are twice as large as the petals of sample A, while the bottom nanorods layer is also about 6 mmin thickness. Sample C has a bilayer nanorods structure built with double nanorods layers. The top nanorods layer of sample C is estimated to be about 8 mm in thickness, while the bottom layer is about 4 mm in thickness. Fig. 1I shows that the tips of nanorods on the top layer of sample C can be identified as two kinds of nanorods: the large one with a tip of 100–200 nm diameter and the small one with a tip of around 50 nm diameter. Fig. 1G and I shows that the small nanorods come from the backbones of large nanorods, which are similar to needle-like branches in the nanodendrite structure (Fig. 1F). Sample C shares the same reaction solution as that of sample B, except for different immersing ways. Sample C is faced downward while sample B is faced upward. Multilayer nanorods array is usually obtained by hydrothermal method when the substrate is faced downward in reaction solution [12,13]. However, both the needle-like branches in sample B and the small nanorods in sample C indicate the secondary nucleation and growth occur in this reaction solution with high ammonia contents, compared to that of sample A. The higher ammonia content of sample B in initial solution could lead to the slight dissolution of the surface on the backbones of nanodendrite due to the amphoteric feature of ZnO materials [14]. Crystal surface with defects tends to further lower their energy through surface reconstruction, which provides active sites for the secondary nucleation [15]. Fig. 2 proposes a possible shape evolution of the bilayer nanodendrite–nanorod structure: (A) seed layer deposition on the substrate; (B) growth of nanorods layer; (C) growth of nanoflower-like top layer; and (D) secondary nuclea- tion and growth from backbones of nanodendrite. Firstly ZnO nanorods grow on the seed layer (Fig. 2A, B) at 90 1C through the reactions of Zn 2+ cations with OH − anions as foolows: NH 3 ⋅H 2 O⇔NH þ 4 þOH − ð1Þ Zn 2þ þOH − -½ZnðOHÞ 4  2− -ZnO þ H 2 O þ OH − ð2Þ Zn 2þ þ NH 3 ⋅H 2 O-½ZnðNH 3 Þ 4  2þ þ H 2 O ð3Þ ZnO þ H 2 O þ OH − -½ZnðOHÞ 4  2− ð4Þ In the reaction system, NH 3 Á H 2 O is partly combined with Zn 2+ and releases OH − slowly, resulting in a relatively stable environ- ment for crystal growth. Due to the sufficient Zn 2+ concentration of the refreshed solution, the flower-like structure was assembled by nanorods on the top of nanorods array [13] (Fig. 2C). According to Eq. (4), excessive alkaline environment in sample B may partially dissolve the surface on the backbones of nanodendrite which provides active sites for the renucleation and sequential growth of nanorods, finally resulting in the formation of complex Fig. 1. (A)—(C) are the side and top-view SEM images of sample A; (D)—(F) are the side and top-view SEM images of sample B; and (G)—(I) are the side and top-view SEM images of sample C. The inset (i), (ii) and (iii) images are over-view SEM images of sample A, B and C, respectively. Y. Lou et al. / Materials Letters 107 (2013) 126–129 127 three-dimensional ZnO nanodendrite–nanorod structure (Fig. 2D). For the sample C, the secondary nucleation and growth occur on the side of the large nanorods, resulting in mixed nanorod structure. Therefore, the ammonia concentration in the initial hydrothermal solution plays the key role for the formation of needle-like nanorods in ZnO nanodendrite–nanorod structure. Fig. 3 shows the photoluminescence spectra of samples A and B which share the same growth condition except for different ammonia contents. The PL spectra consisted of a weak near band-energy UV emission at around 3.2 eV and a broad visible emission in the range of 1.6–2.8 eV. The visible emission which is the surface-state related defect emission [16] can be Gaussian-resolved into three bands located at 1.95 eV, 2.20 eV and 2.39 eV (corresponding to the orange, low- and high-energy green luminescence, respectively). It is believed that oxygen vacancies (V O ) and zinc vacancies (V Zn ) contribute to the green luminescence [17], while oxygen interstitials (O i ) are responsible for the orange luminescence [18]. On comparing the PL spectra, sample B has lower visible emission and higher UV emission, indicating better optical property and fewer defects with respect to sample A. Less defects concentration in sample B indicates secondary nucleation and growth on the surface defects of back- bones which eliminate partial defects, resulting in the low visible emission. 4. Conclusion In summary, nanoflower–nanorod structure, nanodendrite– nanorod structure and double nanorods layers structure were obtained by controlling growth concentration and position via low-temperature hydrothermal process. The shape evolution of nanodendrite–nanorod structure was discussed. The ZnO film with nanodendrite–nanorod structure has better optical properties and lesser defects comparing to those of nanoflower–nanorod struc- tures. High ammonia concentration in the initial hydrothermal solution promotes secondary nucleation and growth on surface defects of backbones, resulting in the formation of needle-like nanorods in ZnO nanodendrite–nanorod structure. Acknowledgment The authors acknowledge the supports of the Shanghai Leading Academic Discipline Project (S30107), National Natural Science Foundation of China (Grant no. 51202138), Natural Science Foun- dation of Shanghai (Grant no. 12ZR1410500), Research and Inno- vation Projects of Shanghai Education Commission (11YZ22), Project of Shanghai Environment Condition (10dz2252300), Science Foundation for Excellent Youth Scholars of Shanghai University, and Science Foundation for Excellent Youth Scholars of Universities (Shanghai). References [1] Yang Q, Liu Y, Pan CF, Chen J, Wen XN, Wang ZL. Nano Lett 2013;13:607–13. [2] Kim A, Won Y, Woo K, Kim CH, Moon J. ACS Nano 2013;7:1081–91. [3] Schlur L, Carton A, Leveque P, Guillon D, Pourroy G. J Phys Chem C 2013;117:2993–3001. [4] Zhang HJ, Wu RF, Chen ZW, Liu G, Zhang ZN, Jiao Z. Crystengcomm 2012;14:1775–82. [5] Suresh V, Jayaraman S, Jailani MIB, Srinivasan MP. J Colloid Interface Sci 2013;394:13–9. 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The shape evolution of nanodendrite–nanorod structure was discussed. The ZnO