Ce-induced single-crystalline hierarchical zinc oxide nanobrushes

Transcription

1 Superlattices and Microstructures 44 (2008) Contents lists available at ScienceDirect Superlattices and Microstructures journal homepage: Ce-induced single-crystalline hierarchical zinc oxide nanobrushes He-Chun Gong, Jia-Fu Zhong, Shao-Min Zhou, Bin Zhang, Zhao-Han Li, Zu-Liang Du Key Lab for Special Functional Materials of Ministry of Education, Henan University, Kaifeng , People s Republic of China a r t i c l e i n f o a b s t r a c t Article history: Received 6 December 2007 Received in revised form 2 April 2008 Accepted 22 April 2008 Available online 2 June 2008 Keywords: ZnO II VI semiconductors Nanobrushes High-density hierarchical brush-like nanostructures of Ce-doped ZnO are first synthesized by a two-step approach (sol gel and chemical vapor deposition). This novel structure, called nanobrush, is a new member in the family of hexagonal crystal ZnO nanostructures, which is composed of two parts: a micronsized prism-like base and nano-sized vertical nanorod arrays. The nanobrush is enclosed by the (0001) and {0110} facets and grows towards the [0001] direction. Room-temperature photoluminescence (RTPL) spectra show a weak UV emission but a strong broad green emission Elsevier Ltd. All rights reserved. 1. Introduction As an important semiconductor, wurtzite ZnO with a direct band gap (E g = 3.37 ev) displays unique features such as a high electron hole binding energy of 60 mev, large piezoelectric and ferromagnetic coefficients with a predicted Curie temperature above room temperature when doped with transition metals [1,2]. Due to its hexagonal wurtzite structure and polar crystal surfaces, ZnO exhibits a large family of nanostructures [1 11]. In addition to the conventional nanowire [3,4] and nanobelt [5,6] structures, many interesting morphologies have recently been discovered, such as nanosprings [7], nanodisks [8], nanobridges [9,14], nanonails [9,10,15], nanotips [11], nanocones [16], and nanocastles [12]. However, these small sizes of the nanoscale building blocks make the fabrication process always costly and complex. Hierarchical nanostructure that has both the properties of nanoscale materials and ease of manipulation, may not only be directly utilized in fabrication Corresponding author. Tel.: address: smzhou@henu.edu.cn (S.-M. Zhou) /$ see front matter 2008 Elsevier Ltd. All rights reserved. doi: /j.spmi

2 184 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) of devices but also facilitate assembly processes as a transition stage in the formation of other nanostructures and provide an attractive alternative to the bottom-up fabrication [13]. It is well-known that doping in semiconductor with selective elements offers an effective approach to adjust the electrical, optical, and magnetic properties crucial in practical applications [17 20]. Rare-earth metal Ce possesses unique optical and catalytic characteristic, therefore Ce may be an ideal dopant and catalyst. Ce-doped ZnO nanobelts and nanowires [3,4] have been reported and our group has synthesized metal doping nanowires [21,22]. However, to date hierarchical Ce-doped ZnO nanobrushes have not been reported. In this paper, we have first switched the growth from hexagonal prisms to nanorod arrays, resulting in the formation of single-crystalline Ce-doped nanobrushes. The morphology is basically composed of two parts: quasi-two-dimensional (Q2D) micron-sized hexagonal crystal base and quasi-one-dimensional (Q1D) nano-sized nanorod arrays on the base. In addition, we discuss the plausible growth mechanism and propose a model for the growth of hexagonal nanobrushes on the Si substrate. 2. Experimental procedure In typical experiment, Zn(NO 3 ) 2 6H 2 O and Ce(NO 3 ) 3 6H 2 O (a molar ratio of about 20:1) were dissolved in deionized water, and appropriated amounts of citric acid, ethylene glycol were then added to form a sol at 80 C for 1 h. Then, the solution was polymerized into a gel at 150 C for 5 h and the gel was pre-pyrolyzed to become an amorphous composite precursor at 400 C for 1 h. The precursor was ground, placed in a ceramic boat and put into a horizontal tube furnace [3,4]. A Si substrate was placed downstream to collect the products. The furnace was heated up to 950 C and kept at this temperature for 60 min. Meanwhile, N 2 was introduced into the tube at a flow rate of 300 sccm. Then the furnace was turned off and the system was quickly cooled. A large amount of ZnO:Ce nanobrushes were obtained on the substrate. The products were analyzed by X-ray diffraction (XRD) (X Pert Pro, Philips, Holland), scanning electron microscopy (SEM) (JSM-5600, JEOL, Japan) with energy dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and transmission electron microscopy (TEM) (JEOL-2010, Japan), as well as TG-DTA (EXSTA R 6000, Japan). The PL (SPEX F212, SPEX Co. America) measurement was performed at room temperature (RT) using a lamp of Xe (λ = 330 nm) as the excitation source. 3. Result and discussion Fig. 1(a) (e) is the SEM images of the as-synthesized products on the Si substrate. A lowmagnification SEM image is shown in Fig. 1(a), revealing that the entire substrate is covered with highdensity hierarchical hexagonal brush-like nanostructures. A medium magnification one in Fig. 1(b) further indicates that the nanobrush is composed of two parts: a large hexagonal columnar base and vertical nano-sized nanorod arrays. The high-resolution SEM images [Fig. 1(c), (d)] show that the diameter and length of the base is respectively about 3 µm and 5 µm, while the diameter and length of the nanorod reaches up to 100 nm and 1.5 µm, respectively. The nanorods and the base share a smooth outer surface and no trace of connection could be observed at the junctions. In addition, many of the nanobrushes are attached to each other at their root [as shown in Fig. 1(e)]. Further, the ZnO nanobrushes are studied using TEM, SAED, and high-resolution TEM (HRTEM). As indicated in Fig. 2(a), a typical TEM image of a single nanobrush is shown. An enlarging TEM image [Fig. 2(b)] demonstrates arrays of rod-like nanostructures with the diameter of 100 nm at the top of the single nanobrush. The corresponding SAED patterns and HRTEM image are shown in Fig. 2(c), (d), in which ZnO nanobrushes are single-crystalline [a lattice fringe (002) with about 0.26 nm] and growth along the [0001] direction are revealed. Fig. 3(a) shows a typical EDX spectrum of the nanobrushes exhibiting signals corresponding to element Zn, O and Ce. The intensity ratios among the Zn, O and Ce peaks suggest that the Ce content in the nanobrushes is 0.74%. Further, high-resolution XPS spectra of the as-synthesized Ce-doped ZnO nanobrushes are shown in Fig. 3(b) (d). The characteristic peaks of oxygen (531.1 ev and ev are respectively for the O 1s at ZnO and the adsorbed oxygen species), zinc (Zn 2p 3/2

3 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) Fig. 1. (a) A low magnification SEM image of the as-synthesized nanobrushes grown on Si substrate; (b) A medium magnification SEM image of the nanobrushes; (c) (e) high magnification SEM images of the nanobrushes. peak at ev) and cerium (Ce 3d 5/2 peak at ev, the feature A denote shake-up of Ce 3+ ) are detected, clearly indicating the existence of the Ce element in the ZnO nanobrushes. The XPS analysis shows that both of Ce and Zn are in the oxide state. According to the area of the peaks, the ratio of Zn, Ce and O in the as-synthesized sample is about 99.26:0.74:100, which may demonstrate that 1:1 (Zn + Ce)/O composition is consistent with stoichiometric (Zn + Ce)/O and compatible with EDX results. In Fig. 3(e), a typical XRD patterns are shown, in which all reflections can be indexed to a crystalline-zno structure with lattice parameters a = 3.26 and c = 5.21, which are near the reported values [3,4]. No signals of other impurities are detected, implying the synthesis of Ce-doped ZnO products. To understand the growth kinetics, the TG-DTA analysis of the Zn, Ce, C, and O precursor is performed under similar environment used for the synthesis of these Ce-doped ZnO nanobrushes [Fig. 4(a)]. From the TG-DTA curve, three loss weight processes are observed. In the initial stage, the loss weight attributes to the vaporized H 2 O of the precursor. From 400 C to 800 C, the weight loss of about 30 wt% is due to the C atoms reducing composite oxides into Zn (Ce). Above 800 C, a sharp loss of about 40 wt% could be assigned to evaporating most of Zn (Ce) species. Thus, with the increase of the temperature, most of the Zn (Ce) is shortly vaporized whereas at constant temperature the loss weight is very little, which leads to rapid decrease of the Zn (Ce) vapor concentration. Base on the TG-DTA data, the morphology of ZnO nanostructures are investigated. When the temperature reached 950 C, the furnace was turned off and quickly cooled down, and then the hexagonal prismlike ZnO product was obtained as shown in Fig. 4(b). Up to 950 C, the temperature was held for 10 min before the furnace was turned off and quickly cooled down. Immature or intermediate ZnO brush-like products were observed due to the insufficient supply of Zn (Ce) [see a typical SEM image in Fig. 4(c)]. These results imply that the switch of morphology is induced by the Zn (Ce) vapor concentration change.

4 186 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) Fig. 2. (a) A low magnification TEM image of a nanobrush; (b) An enlarged TEM image of nanorod arrays on the tip of a nanobrush; (c) SAED patterns recorded on the nanorod; (d) An HRTEM image showing the ZnO lattice taken from the nanorod. ZnCeO could be simply formed by a chemical reaction; however, the formation of a singlecrystalline Ce-doped nanobrush would involve a very complicated process. Since the radius and the valence of the Ce 3+ are different from those of Zn 2+ ions, it is very difficult for Ce 3+ to be incorporated into ZnO matrices and substitute for Zn 2+ ion site in general if the raw materials are vaporized by a simple mixing method. In this experiment, we used a solid-solution precursor (by the sol gel method) as the starting material, in which Zn, Ce, C, and O can reach a mix on an atomic scale [3,4]. In the undoped sample, a few of microparticles and microrods [as shown in Fig. 4(d)] are obtained on the surface of the substrate, but no nanobrushes. Based on the experimental results, we propose a model for the growth of the hexagonal nanobrushes. As shown in Fig. 5(a) (f), a nanobrush is surrounded by the six equivalent {0110} facets and its growth direction is along the [0001]. Briefly, under Ce catalysis the carbon atoms can directly reduce composite oxides into Zn (Ce) and form counterpart vapors. A part of Zn (Ce) vapor transfers into the low temperature region before condensing into alloy liquid droplets (littler Ce + Zn) and recombining with oxygen to form Ce-doped ZnO nucleus (for a base growth) as indicated in Fig. 5(a) (c). During the growth of the base, the alloy liquid droplet acts as a catalyst. Based on the morphology of the nanobrushes (Fig. 1), it is believed that the nanobrushes are formed in two steps: (1) growth of the micron-sized bases initially on Si substrate and (2) growth of the nanorod arrays on the top of the bases. In the initial stage, the abundant reactants provide sufficient Zn vapor; therefore, the big hexagonal base is rapidly formed in a short time as shown in Fig. 5(d). From the TG-DTA curve, we can see that the vapor concentration quickly drops after staying at 950 C for 10 min. When the concentration of Zn vapor drops to a certain threshold, the growth of the base halts [Fig. 4(b)]. However, the vapor concentration would be the supersaturation level and new nucleus would be induced on the top of the hexagonal base as shown in Fig. 5(e). The corresponding microstructure of this stage is shown in Fig. 4(c), which confirms our growth mechanism [11,12]. The

5 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) Fig. 3. (a) EDX spectrum of the as-synthesized Ce-doped ZnO nanobrushes; XPS spectra obtained from the Ce-doped ZnO nanobrushes (b) O1s, (c) Zn 2P 3/2 and (d) Ce 3d 5/2 ; (e) XRD patterns take from the Ce-doped ZnO nanobrushes. alloy liquid droplets act as the new Ce-doped ZnO nucleus and then further grow into the nanorod arrays on the base [Fig. 5(f)], which is similar to previous reports [23,24]. Fig. 6 shows the PL results of Ce-doped and undoped ZnO nanostructures measured using Xe light as the excitation source (330 nm) at room temperature. From the PL spectra, it can be seen that undoped ZnO nanostructures and Ce-doped ZnO nanobrushes exhibit a peak at ev and ev, corresponding to the near band edge (NBE) peak originated from the recombination of

6 188 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) Fig. 4. (a) TG-DTA curve of the precursors under similar environment used for the synthesis of ZnO nanobrushes; (b) and (c) two typical SEM images of nanobrushes in their different growth stages and a high-magnification SEM image [inset in Fig. 4(c)]; (d) A typical SEM image of the undoped ZnO nanostructures. the free excitons of ZnO. When Ce ions are incorporated into ZnO and become donors, the bandgap structure of ZnO nanobrushes is modulated substantially and new emission centers are formed, which leads to the lower-energy shift of the NBE peak [25,26]. In the visible range, the intensity of the green emission of Ce-doped ZnO increases compared with that of the undoped ZnO and a red-shift appears in Ce-doped ZnO samples. This could be attributed to the existence of Ce impurities in ZnO nanostructures, which ought to give rise to some new defects, such as Ce vacancies. The enhanced intensity of the green emission band confirms the increased number of defect sites [27]. 4. Conclusions In summary, large-quantity single-crystalline Ce-doped ZnO nanobrushes are synthesized by sol gel combined with chemical vapor deposition, in which corresponding formation mechanism would be a self-catalyzed VLS growth process. The growth technique can be applied to fabricate other hierarchical doping semiconductors. The synthesis of this new nanobrush morphology could enrich our knowledge about wurtzite crystal growth, and it presents a new structural configuration with potential applications in photoelectric nanodevices. Acknowledgments This work was sponsored by Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 2008HASTIT002) and partially supported by the Ministry of Science and Technology of China (973Grant No. 2007cb616911).

7 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) Fig. 5. Schematic steps of the formation of the ZnO nanobrushes. Fig. 6. RTPL spectra of two samples measured using Xe light. Inset shows RTPL spectra of UV region.

8 190 H.-C. Gong et al. / Superlattices and Microstructures 44 (2008) References [1] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) [2] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) [3] B.C. Cheng, Y.H. Xiao, G.S. Wu, L.D. Zhang, Appl. Phys. Lett. 84 (2003) 416. [4] B.C. Cheng, Y.H. Xiao, G.S. Wu, L.D. Zhang, Adv. Funct. Mater. 14 (2004) 913. [5] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) [6] S. Choopun, N. Hongsith, P. Mangkorntong, N. Mangkorntong, Physica E 39 (2007) 53. [7] X.Y. Kong, D. Yong, R. Yang, Z.L. Wang, Science 303 (2004) [8] P.X. Gao, C.S. Lao, D. Yong, Z.L. Wang, Adv. Funct. Mater. 16 (2006) 53. [9] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235. [10] G.Z. Shen, J.H. Chao, J.K. Yoo, G.C. Yi, C.J. Lee, J. Phys. Chem. B 109 (2005) [11] C.X. Xu, X.W. Sun, Appl. Phys. Lett. 83 (2003) [12] X.D. Wang, J.H. Song, Z.L. Wang, Chem. Phys. Lett. 424 (2006) 86. [13] H. Wang, X.H. Zhang, X.M. Meng, S.M. Zhou, S.K. Wu, W.S. Shi, S.T. Lee, Angew. Chem. Int. Ed. 44 (2005) [14] J.S. Lee, M.S. Islam, S. Kim, Nano Lett. 6 (2006) [15] S. Kar, A. Dev, S. Chaudhuri, J. Phys. Chem. B 110 (2006) [16] X.H. Han, G.Z. Wang, J.S. Jie, W.C. Choy, Y. Luo, T.I. Yuk, J.G. Hou, J. Phys. Chem. B 109 (2005) [17] S.Y. Bae, C.W. Na, J.H. Kang, J.H. Park, J. Phys. Chem. B 109 (2005) [18] R.K. Zheng, H. Liu, X.X. Zhang, V.A. Roy, A.B. Djurisic, Appl. Phys. Lett. 85 (2004) [19] V. Bhosle, A. Tiwari, J. Narayan, J. Appl. Phys. 100 (2006) [20] S. Sadhu, T. Sen, A. Patra, Chem. Phys. Lett. 440 (2007) 121. [21] S.M. Zhou, X.H. Zhang, X. Meng, S.T. Lee, Nanotechnology 15 (2004) [22] S.M. Zhou, X.H Zhang, X. Meng, S.T. Lee, Phys. Status Solidi A 202 (2005) 405. [23] R.C. Wang, C.P. Liu, J.L. Huang, S.J. Chen, Appl. Phys. Lett. 86 (2005) [24] H.B. Lu, L. Liao, H. Li, D.F. Wang, J.C. Li, Q. Fu, B.P. Zhu, Y. Wu, Physica E (2008), doi: /j.physe [25] Y. Kong, D. Yu, B. Zhang, W. Fang, S. Feng, Appl. Phys. Lett. 78 (2001) 407. [26] Y. Jun, S. Jung, G. Yi, Appl. Phys. Lett. 82 (2003) 964. [27] S.Y. Yae, H.C. Choi, C.W. Na, J. Park, Appl. Phys. Lett. 86 (2005)