Twinned Zn 2 TiO 4 Spinel Nanowires Using ZnO Nanowires as a Template**

Transcription

1 DOI: /adma Twinned Zn 2 TiO 4 Spinel Nanowires Using ZnO Nanowires as a Template** By Yi Yang, Xiao Wei Sun,* Beng Kang Tay, Jian Xiong Wang, Zhi Li Dong, and Hai Ming Fan COMMUNICATION One-dimensional (1D) semiconductor metal oxide nanostructures, such as nanowires, nanobelts, and nanotubes, have attracted considerable attention due to their prominent electrical, optical, and chemical properties and potential applications in nanoscale electronic and optoelectronic devices. [1 5] However, the properties and applications of those nanomaterials are limited by their simple binary systems. [6] Therefore, synthesis of complex functional nanomaterials with controlled size and morphology, such as core/shell quantum dots, [7] nanowires and nanobelts, [8 11] heterostructures, [6,12] superlattices, [13] and nanotubes, [14 16] are highly desirable. Furthermore, 1D ternary nanocomposites, such as Zn 2 SnO 4, [8 10,17] Zn 2 TiO 4, [18] ZnGa 2 O 4, [11,16] MgAl 2 O 4, [14] CoFe 2 O 4, [19] exhibit specific functions that are unattainable by common binary composites. Conventional synthesis methods of those ternary composites usually involve high-energy ball milling of two binary solid oxides [18] or a high-temperature solid reaction, [20,21] which are not suitable for 1D nanostructure growth. Only recently, syntheses of single-crystal ternary oxide nanowires and nanotubes have been achieved by using Ga 2 O 3, MgO, or ZnO nanowires as template. [11,14,15] For face-centered cubic (fcc) nanostructures, twinning is probably the most common structural defect, especially in metal and metal alloy nanoparticles, [22,23] where two subcrystallites share a facet to form a mirror image of each other. Twinning is occasionally observed in binary systems, such as cubic ZnS and InP nanowires, [24,25] both of which have a preferential <111> growth direction. However, reports on the synthesis of twinned structures of fcc ternary nanowires are rare; [10] that is, the formation of twinned nanowires has yet to be revealed. [*] Prof. X. W. Sun, Y. Yang, Prof. B. K. Tay, Dr. J. X. Wang School of Electrical and Electronic Engineering Nanyang Technological University Nanyang Avenue (Singapore) exwsun@ntu.edu.sg Prof. Z. L. Dong School of Materials Science and Engineering Nanyang Technological University Nanyang Avenue (Singapore) Dr. H. M. Fan Department of Physics National University of Singapore Blk S12, 2 Science Drive (Singapore) [**] Sponsorship from a Research Grant Manpower Fund of Nanyang Technological University (RGM 21/04) and a Science and Engineering Research Council Grant from the Agency for Science, Technology and Research (A*STAR) (# ), Singapore, is gratefully acknowledged. Zinc titanate (Zn 2 TiO 4 ) is an inverse spinel, which has been used as a catalyst and pigment in industry. It is one of the leading regenerable catalysts and has been demonstrated to be a good sorbent for removing sulfur-related compounds at high temperature. [26,27] As a dielectric material, its physical, electrical, and optical properties have been studied for various applications. [21,28,29] In this Communication, we shall demonstrate a three-stage synthesis of twinned Zn 2 TiO 4 nanowires using ZnO nanowires as template, which could be further developed for controlled synthesis of ternary oxide nanostructures. Figure 1a c shows the scanning electron microscopy (SEM) images of ZnO nanowires and ZnO/Ti core/shell nanowires before and after annealing, respectively. The as-grown ZnO nanowires shown in Figure 1a are randomly aligned, of about nm in diameter, and 10 lm in length. It can be seen clearly that the morphology of the ZnO nanowires is reserved after coating with Ti (Fig. 1b). The surface is smooth and the diameter of the Ti-coated nanowires is increased to nm. No aggregated Ti particles could be seen in Figure 1b, indicating a uniform deposition of Ti on ZnO nanowires. After thermal annealing in low vacuum (0.1 Torr; 1 Torr = 133 Pa) at 800 C for 8 h, the nanowires are bent and curved, with small perturbations observed on the surface (Fig. 1c). Figure 1d shows the X-ray diffraction (XRD) patterns for the ZnO nanowires (Curve A), and the Ti-coated ZnO nanowires before (Curve B) and after (Curve C) annealing. All peaks in Curve A can be indexed to a wurtzite ZnO crystal with lattice constants of a = Å and c = Å [Joint Committee on Powder Diffraction Standards (JCPDS) Card No ]. It can be seen from Curve B that the ZnO crystal structure is reserved after Ti coating but before thermal annealing; no new peak or peak shift is observed. Moreover, no Ti peaks could be identified in Curve B. Therefore, either the Ti phase is amorphous or the diffraction signal of crystalline Ti is too low to be detected. It can be seen from Curve C that phase transformation of Ti-coated ZnO nanowires happened after thermal annealing; four new peaks at 29.9, 35.2, 53.0, and 62.0 indicated by asterisks can be clearly seen, which correspond respectively to (200), (311) (strongest), (422), and (440) of the fcc crystal structure of spinel Zn 2 TiO 4 (space group Fd-3m with a lattice constant of a = Å, JCPDS Card No ). It is worth mentioning that the atomic ratio of Ti/Zn should be 1:2 to form stoichiometric Zn 2 TiO 4. We have optimized the sputtering condition to yield a maximum quantity of nanowires with this desired Ti/Zn ratio. It can be seen from Figure 1d, no diffraction peak Adv. Mater. 2007, 19, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1839

2 COMMUNICATION (d) (degrees) Figure 1. Scanning electron microscopy (SEM) images of a) the asgrown ZnO nanowires, b) Ti-coated ZnO/Ti core/shell nanowires, and c) Zn 2 TiO 4 spinel nanowires, including a high-magnification SEM image of a Zn 2 TiO 4 nanowire (inset). d) Corresponding X-ray diffraction patterns of (a c), denoted as curves A C, respectively. from either ZnTiO 3 or TiO 2 secondary phases could be found. The residual ZnO diffractions in curve C, which have been weakened in intensity compared to curves A and B, come from the ZnO nanowires at the bottom of the sample where no Ti was deposited. The obtained nanowires were scratched off after Ti coating and thermal annealing to investigate the crystal structures and phase transformation by transmission electron microscopy (TEM). Figure 2a and b shows the bright- and dark-field TEM images, respectively, of a representative Ti-coated ZnO nanowire. High-resolution TEM (HRTEM) images of the ZnO core (indicated by a rectangle) and Ti shell (indicated by a circle) in Figure 2a are shown in Figure 2c and d, respectively. As shown in Figure 2a and b, the distribution of the ZnO and Ti phase (a core/shell structure) can be clearly distinguished from the dark field image. The thickness of the Ti shell varies from about 40 nm to a few nanometers from the left to the right side of the nanowire. This is reasonable because only half of the nanowires were facing the sputtering target. The amorphous Ti layer deposited in this way was not dense, as can be seen from the inhomogeneous contrast. This result agrees with Yu s work, [30] where a purely amorphous Ti metal film was sputter-deposited on a cooled substrate. It can be seen that the ZnO nanowire was partially bombarded off by electrons and ions in the plasma, which is a common phenomenon in sputtering, resulting in ZnO content fluctuation along the nanowire. The inset in Figure 2a shows the selected area electron diffraction (SAED) pattern of the Ti-coated ZnO structure. It contains diffraction patterns of wurtzite ZnO taken along the [21 1 0] zone axis and a heavily amorphous phase of Ti mixed with small fractions of hexagonal close-packed crystallites, where the three major diffraction rings match Ti {101 0}, {101 1}, and {112 0}. Structurally, ZnO has three fast growth directions: [0001], <21 1 0> and <011 0>. Correspondingly, the nanowires and nanobelts grown along [0001] and <011 0> or <21 1 0> directions have hexagonal and rectangular cross sections, respectively. [5] In this work, the ZnO nanowires elongate along the [011 0] direction, as revealed by the SAED pattern, which is similar to nanobelts. The smooth core/shell interfaces shown in Figure 2c exhibit distinguishable solid solid contrast, as pointed out by the arrows in Figure 2a. HRTEM of the Ti layer deposited at room temperature is shown in Figure 2d, where the Ti crystallite phase could hardly be seen. It is worth mentioning that a small amount of nanowires appeared without Ti coating; this was probably from the ZnO nanowires at the bottom of the sample, consistent with the XRD results. By thermal annealing at 800 C under low vacuum, the solid-state reaction that involves the diffusion of Ti into the ZnO crystal happens, resulting in a phase transformation from wurtzite ZnO to spinel Zn 2 TiO 4. The final product contains a large amount of twinned nanowires. Figure 3a shows a typical twinned Zn 2 TiO 4 spinel projected along the [011] zone axis. It can be seen that the twinned nanowire is composed of large parallelogram-shaped subcrystallites, tens of nanometers in width. Some much smaller ones are less than 10 nm wide, which can be defined as the grain-transition region between two large grains. The corresponding SAED pattern shown in the inset of Figure 3a reveals the (111) twin structure, with the nanowire growing along the [1 1 1] direction, which is consistent with twinned spinel Zn 2 SnO 4 reported by Chen WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19,

3 (c) Figure 2. a,b) Bright- and dark-field TEM images, respectively, of a typical amorphous Ti-coated ZnO core/shell nanowire. The inset in (a) shows the selected area electron diffraction (SAED) pattern of the nanowire. c,d) High-resolution TEM (HRTEM) images of the single-crystal ZnO core and amorphous Ti shell, respectively, regions (indicated by a rectangle and a circle in (a), respectively) of the nanowire. et al. [10] The twin plane is parallel to (1 1 1) or (111 ) T, where the subscript T denotes twin reflection to distinguish from single crystal. Our twinned nanowires present a zigzag feature, [10] which can be attributed to the <111>-oriented growth direction of each individual grain. The HRTEM images of the grain and twin boundary (TB) taken from Figure 3a are shown in Figure 3b and c, respectively. Both figures reveal good crystal quality of the nanowire. The interplanar spacings of 0.49 and 0.42 nm perfectly match the d 111 and d 200 spacings, respectively, of Zn 2 TiO 4 crystal. The fast Fourier transform (FFT) pattern of Figure 3b (inset A) is in good agreement with the spinel crystal structure of Zn 2 TiO 4 taken along the [011] zone axis. The simulated HRTEM image (inset B in Fig. 3b) was obtained from the Inorganic Crystal Structure Database (ICSD) #80850 for a sample thickness of 26 nm and a defocus value of 80 nm. Inset C in Figure 3b is a schematic diagram (simulation from ICSD #80850) of a possible Zn 2 TiO 4 crystal structure, projected along [110] axis, superimposed on the enlarged experimental HRTEM image. Four atoms are in the base with Zn (1/8,1/8,1/8), Zn/Ti (1/2,1/2,1/2) (equal site occupation factor), and O (0.260,0.260,0.260), which are denoted as black, white, and small black dots, respectively. Both the simulated HRTEM image and crystal structure agree well with the experimental HRTEM image, as indicated by the diamond shapes in insets B and C of Figure 3b. The TB marked by a dashed line in Figure 3c can be clearly resolved, showing that the boundary is free of misfit or distortion. The FFT pattern of the TB (inset in Fig. 3c) agrees well with the SAED pattern of the twinned nanowire. The twinning angle across the boundary is measured to be about 141, and the relative rotational angle from FFT pattern (inset in Fig. 3c) is about 70.2, consistent with theoretical values of 141 and 70.5, respectively. Figure 3d shows the smallest grain width observed, which is around 4 nm wide (the two TBs are marked by dashed lines). Lattice distortion across the boundary can be seen. Figure 4a shows the TEM image of another twinned spinel nanowire with zone axis tilted by 90 relative to previous one observed along [011] (Fig. 3a), where the projected grain images are rectangular-shaped. As a general feature of a twinned spinel structure, the nanowire is elongated along [1 1 1] direction, whereas both (022) and (02 2 ) facets of the nanowire are 90 away from the TB, with no zig-zag morphology or relative grain shift observed. The parallel TBs can be clearly seen, and the subcrystallite size distribution is similar to the one presented in Figure 3a. Figure 4b is the HRTEM image of the TB taken from the rectangular area in Figure 4a, and inset A shows the corresponding FFT pattern. The lattice spacing for the (1 1 1) and (022) planes are 0.49 and 0.30 nm, respectively, consistent with our previous TEM analysis in Figure 3b (0.49 nm corresponding to d 111 spacing). Inset B shows the schematic diagram of the enlarged crystal structure of Zn 2 TiO 4 projected along the [21 1] direction, and the denotation is the same as that of Figure 3b. It can be seen that the twinned structure feature is absent along the [21 1] zone axis, therefore, the TB in HRTEM can only be distinguished from the contrast. From a chemical point of view, formation of twinned spinel Zn 2 TiO 4 nanowires is a solid-state reaction process, in which the ZnO nanowires act as a template and spatially confine the reaction within them. Figure 5 illustrates the formation mechanism of the twinned Zn 2 TiO 4 nanowires. Firstly, an amorphous Ti layer is deposited on the ZnO nanowires (Fig. 5a). Upon thermal annealing of the ZnO/Ti core/shell structure at high temperature, Ti metal may absorb oxygen from the ambient and ZnO to form TiO x through a chemical reaction, as illustrated in Figure 5b. The amorphous Ti metal is chemically active at high temperature, [31] which is supported by the fact that the enthalpies of TiO 2 (DH TiO2 = 944 kj mol 1 ) and TiO COMMUNICATION Adv. Mater. 2007, 19, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 COMMUNICATION Figure 3. a) Low-magnification TEM image of a typical twinned Zn 2 TiO 4 nanowire. The corresponding SAED pattern shown in the inset reveals the (111) twin structure, with the nanowire growing along the [1 1 1] direction. b) HRTEM image of a subcrystallite. Inset A shows the corresponding fast Fourier transform (FFT) pattern. Inset B shows a simulated HRTEM image obtained from the Inorganic Crystal Structure Database (ICSD) #80850 for a sample thickness of 26 nm and a defocus value of 80 nm. Inset C shows a schematic diagram (simulation from ICSD #80850) of a possible Zn 2 TiO 4 crystal structure projected along [110] axis, where black, white, and small black dots denote Zn, Zn/Ti (equal site occupation factor), and O, respectively, superimposed on the experimental HRTEM image. c) HRTEM image of a twin boundary (TB) between two large subcrystallites and the corresponding FFT pattern as inset. d) HRTEM image showing the smallest grain of 4 nm wide found between two TBs. (DH TiO = 519 kj mol 1 ) are much smaller than that of ZnO (DH ZnO = 350 kj mol 1 ). [31] In comparison with Fan s work, [14,15] our TEM analysis showed no voids or tube morphology; therefore, the diffusion speed of Ti is faster than or comparable with those of Zn 2+ and O 2. As Ti metal diffuses into ZnO, both Zn 2+ and O 2 cannot stay in their original lattice sites. Eventually, Zn 2 TiO 4 spinel crystallites are formed with their preferred orientation in <111>. For an fcc structure, the surface energy of different facets follows {111} < {100} < {110}, [22] therefore, {111} is the dominant facet that encloses the crystal, which is consistent with our TEM observations of the twinned Zn 2 TiO 4 nanowire. The twinned crystals can be easily formed when two adjacent {111} faces meet by a relative shear movement of the subcrystallites in the <21 1> direction to produce a symmetrical arrangement, as illustrated in Figure 5c. The shear movement is driven by dislocation-induced stress/strain to minimize the overall system energy. [32] This proposal is reasonable because small and individual subcrystallites can be formed easily in the nanowire due to the fluctuation of ZnO content, as can be seen in Figure 2a, in contrast to that of the Ga 2 O 3 /ZnO core/shell structure reported by Chang et al. [11] Alternatively, the twinned structure can also be formed when new subcrystallites start to grow in a limited space between two existing grains, which most likely happens during the final stage, as illustrated in Figure 5d. In conclusion, we have demonstrated a three-step method to synthesize ternary twinned Zn 2 TiO 4 nanowires. The ZnO nanowires used as template were firstly coated with amorphous Ti to form a ZnO/Ti core/shell structure, followed by annealing at 800 C for 8 h. The ZnO core content appears to be fluctuating along the nanowire after Ti coating. The final product was (111)-twinned Zn 2 TiO 4 nanowires growing along the <111> direction. The formation of the twinned structure involves a solid-state reaction between Ti and the ZnO single crystal, where formation of twinning reduces the system energy. It is possible to apply this technique to synthesize other ternary materials WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19,

5 (a) (b) Ti atoms ZnO nanowire Ti atoms Annealing Zn 2 TiO 4 subcrystallites Ti COMMUNICATION Zn 2+ O 2- (c) [211] (d) Twinned Zn 2 TiO 4 nanowire Figure 4. a) Low-magnification TEM image of a Zn 2 TiO 4 twinned nanowire projected along the [112] zone axis. The inset shows the corresponding SAED pattern. b) HRTEM image of TBs taken from the rectangular selected area in (a). Inset A shows the corresponding FFT pattern. Inset B shows a schematic diagram of the crystal structure, where black, white, and small black dots denote Zn, Zn/Ti (equal site occupation factor), and O, respectively. Experimental Growth of ZnO Nanowires: The ZnO naonwires were synthesized by vapor-phase transport (VPT) from ZnO (99.99%, AlfaAesar) and graphite powder (99.99%, Aldrich) in a ratio of 1:1 [wt %] on a (111)Si substrate (with a 5 nm Au layer serving as catalyst). In brief, a small quartz tube, containing the source and Si substrate, was inserted into a horizontal quartz tube furnace at 950 C. The furnace was kept at about 1 Torr with a rotary pump, with a constant flow of 150 sccm Ar and 5 sccm O 2 for 30 min, after which the small quartz tube was pulled out and cooled down to room temperature. A layer of lightblue product was found on the substrate. Fine ZnO nanowires with a certain density (much smaller than the visible wavelength) can be treated as a homogenous media with an equivalent refractive index [33]. For a reflective substrate like silicon, constructive interference could happen for the film to appear as a certain color, depending on the thickness. Magnetron Sputter Deposition of Ti: The as-synthesized ZnO nanowires were then transferred into a vacuum-deposition system for Ti deposition by DC magnetron sputtering [34]. The deposition took place at room temperature with a base pressure below Torr and a constant Ar flow of 40 sccm. The DC voltage used in the DC magnetron sputtering was 450 V, and the sputtering time was 10 min. The film thickness was controlled to be less than the typical thickness of a ZnO nanowire to meet the stoichiometric requirement of spinel phase. After deposition, the color of the substrate changed from light blue to black. Solid-State Reaction and Phase Transformation: In order to transform the ZnO/Ti core/shell structure into zinc titanate, the sample Figure 5. Schematic illustration of the formation process of twinned Zn 2 TiO 4 nanowires. a) Ti is coated on the surface of the nanowire. b) Cross section of the core/shell nanowire showing Ti on the surface diffusing into ZnO. c) Formation of a TB by two adjacent Zn 2 TiO 4 grains with different orientations moving relatively into a symmetrical arrangement. d) Formation of the twinned Zn 2 TiO 4 nanowires is completed with new Zn 2 TiO 4 grains growing from limited space between two large grains. was then loaded in a program-controlled tube furnace. After evacuating the quartz tube to below 0.1 Torr, an optimized three-stage annealing process was performed under precise control of the reaction temperature under vacuum. The furnace was first slowly heated to 800 C at a rate of 10 C min 1, then maintained for 8 h, followed by slowly cooling down overnight. Characterization: The crystal structure and morphology of the nanowires were investigated by 2h-scan X-ray diffraction (XRD, Siemens D5005, operated at 40 kv), field-emission scanning electron microscopy (FESEM, JSM-6340F, operated at 5 kv), and transmission electron microscopy (TEM, JEM-2010, operated at 200 kv). Received: February 4, 2007 Revised: March 22, 2007 Published online: June 18, 2007 [1] M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, P. D. Yang, Science 2001, 292, [2] J. T. Hu, T. W. Odom, C. M. Lieber, Acc. Chem. Res. 1999, 32, 435. [3] J. X. Wang, X. W. Sun, Y. Yang, H. Huang, Y. C. Lee, O. K. Tan, L. Vayssieres, Nanotechnology 2006, 17, [4] J. X. Wang, X. W. Sun, A. Wei, Y. Lei, X. P. Cai, C. M. Li, Z. L. Dong, Appl. Phys. Lett. 2006, 88, [5] Z. L. Wang, J. Phys.: Condens. Matter 2004, 16, R829. [6] R. R. He, M. Law, R. Fan, F. Kim, P. D. Yang, Nano Lett. 2002, 2, [7] B. O. Dabbousi, J. Rodriguez Viejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, K. F. Jensen, M. G. Bawendi, J. Phys. Chem. B 1997, 101, [8] J. X. Wang, S. S. Xie, H. J. Yuan, X. Q. Yan, D. F. Liu, Y. Gao, Z. P. Zhou, L. Song, L. F. Liu, X. W. Zhao, X. Y. Dou, W. Y. Zhou, G. Wang, Solid State Commun. 2004, 131, 435. Adv. Mater. 2007, 19, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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