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Zn2TiO4-ZnO Nanowire Axial Heterostructures Formed by Unilateral Diffusion Chun Cheng, Wei Li, Tai-Lun Wong, Kin Ming Ho, Kwok Kwong Fung, and Ning Wang* Department of Physics and the William Mong Institute of Nano Science and Technology, The Hong Kong UniVersity of Science and Technology, Hong Kong, China ReceiVed: August 9, 2010; ReVised Manuscript ReceiVed: October 29, 2010
We present a two-stage fabrication method for Zn2TiO4-ZnO nanowire axial heterostructures using the sitespecific deposition of TiO2 on the tips of ZnO nanowires, followed by an annealing process. As revealed by the electron microscopy investigation, the cubic spinel Zn2TiO4 single crystal heads possess two kinds of oriented relationships with ZnO nanowires after high-temperature reaction. The reaction between TiO2 nanoparticles and ZnO nanowires shows a unilateral diffusion characteristic of ZnO into TiO2 which provides a useful method for rational design and fabrication of nanostructures based on nanostructured phase transformation. 1. Introduction ZnO nanostructures have received increasing attention over the past few years because of their prominent structural, optical, and optoelectronic properties.1 In most cases, ZnO-based compounds are technologically more important since they can exhibit specific functions that are unachievable by pure ZnO phase.2 ZnO can form spinel type ternary compounds with other oxides in the forms of ZnM[3+]2O4 or Zn2M[4+]O4 (M ) Al, Cr, Fe, Ga, In, Sn, Sb, Ti, Mn, V) through solid-state reactions by thermal annealing, hydrothermal growth, and in situ vaporliquid-solid (VLS) growth by coevaporating powders of the constituent oxides. Due to the large surface-to-volume ratio and surface symmetry breaking, nanoscale spinel oxides have shown interesting physical properties different from their bulk counterparts. Most of these ternary oxides are wide band gap semiconductors and typical phosphorus materials. In addition, a large diversity of morphologies in nanoscale spinel oxides, such as tubes,3 wires,4 and periodically zigzagged and/or twinned wires,5,6 have been demonstrated. The growth process of classical spinel oxides involves Wagner’s cation counter diffusion mechanism7 via cation migration through the reaction interface in opposite directions with the oxygen sublattice remaining essentially fixed. Another growth mechanism involves the diffusion of both oxygen and cations into the counterparts in an unilateral transfer into the spinel.8 This means that an inert marker plane located at the initial interface will be observed at the ZnO/spinel interface for the ZnO-Al2O3 reaction, while in the case of the MgO-Al2O3 reaction, the marker plane is within the spinel layer. However, reports on the growth mechanism of spinel nanostructures by solid-state reaction are rare due to the practical difficulty in studying the position of the marker planes, especially in nanoscale.9 Zinc titanate (Zn2TiO4), an inverse spinel, has been widely used as a pigment and catalyst in industry. It is one of the preeminent regenerable catalysts and has been shown as an excellent absorbent to remove sulfur-related compounds at high temperatures.10 Recently, the synthesis of crystalline Zn2TiO4 nanowires (NWs) have been achieved using ZnO NWs as the * To whom correspondence should be addressed. E-mail: Phwang@ ust.hk.
templates and coating ZnO NWs with Ti/TiO2.5,6 Using similar coating techniques, different ZnO-based ternary compound nanotubes and NWs such as ZnAl2O4, ZnFe2O4, ZnGa2O4, Zn2SnO4, and Zn2SiO4 have been fabricated.2 Recently we developed a method for site-specific deposition of amorphous TiO2 (a-TiO2) particles on the tips of ZnO NWs.11 The growth process and formation mechanism of a-TiO2-ZnO NW hybrid structures have been discussed in our previous report.11 In this paper we demonstrate Zn2TiO4-ZnO NW heterostructures fabricated by annealing the a-TiO2-ZnO structures. The spinel growth progress of the Zn2TiO4-ZnO NW heterostructures has been investigated by tracing the marker plane positions. We found that the reaction between TiO2 nanoparticles and ZnO NWs involved unilateral diffusion of ZnO into TiO2. 2. Experimental Section 2.1. Growth of ZnO NW Arrays. An alumina boat containing 3 g of ZnO powder was placed in the center of a tube furnace. Si substrates with carbon coating were placed downstream for the nucleation and growth of ZnO NWs. The furnace was heated to 1300 °C and kept for half an hour under vacuum conditions (∼10-2 torr). ZnO NW arrays were found to vertically grow on the substrates when the temperature was about 600-800 °C.12 2.2. Formation of Zn2TiO4-ZnO Heterostructures. One gram of anatase TiO2 nanoparticles was treated with a 10 M NaOH aqueous solution in a Teflon vessel at 150 °C for 12 h. The final TiO2 nanotube product was washed with DI water several times and dried at 80 °C in air.13 TiO2 nanotubes (200 mg), 5 mL of 0.1 M zinc acetate ethanol solution, and 35 mL of 0.5 M NaOH ethanol solution were mixed to form a suspension solution. Then the ZnO NW arrays were placed into the mixed solution and heated at 180 °C for 24 h.11 We found that TiO2 nanotubes were easily dissolved in the mixed solution. This was because of the special structure of TiO2 nanotubes formed by rolling up TiO2 single sheets which might be more reactive than TiO2 crystals. In this mixed solution, the deposition of TiO2 always occurred at the ends of ZnO nanowires. The as-prepared a-TiO2-ZnO heterostructured arrays were annealed at 800 °C in the air for 2 h to convert the TiO2 nanoparticles to Zn2TiO4 crystals. The annealing experiments were also carried
10.1021/jp107513a 2011 American Chemical Society Published on Web 12/10/2010
Zn2TiO4-ZnO Nanowires
Figure 1. SEM images of (a) ZnO NW arrays, (b) a-TiO2-ZnO heterostructured NW arrays, and (c) Zn2TiO4-ZnO heterostructured NW arrays. (d) Schematic illustration of the fabrication process of Zn2TiO4-ZnO heterostructured NW arrays.
out at 650, 700, and 800 °C for 10 min in order to study the formation progress of Zn2TiO4-ZnO heterostructures. The as-grown NWs were characterized by a Philips scanning electron microscope (SEM, XL-30), and a JEOL high-resolution transmission electron microscope (HR-TEM, 2010F) equipped with an energy-dispersive X-ray spectrometer (EDX). Selected area electron diffraction (SAED) was carried out on a Philips CM120 TEM at 120 kV. 3. Results and Discussion The fabrication of Zn2TiO4-ZnO NW axial heterostructures consists of two steps: (1) the site-specific deposition of a-TiO2 on ZnO NW arrays and (2) the annealing treatment to form the cubic spinel Zn2TiO4 as illustrated in Figure 1d. The typical morphologies of the products in each step are shown in Figure 1a-c. The original vertically aligned ZnO NWs are about 100-300 nm in diameter and several micrometers in length. After first treatment, a-TiO2 nanoparticles have been found to attach at the tips of the ZnO NWs (Figure 1b).11 After the annealing treatment, the morphologies of the NWs have not been dramatically changed (Figure 1c).
J. Phys. Chem. C, Vol. 115, No. 1, 2011 79 Figure 2 shows the TEM study of a single ZnO NW with an a-TiO2 nanoparticle at its end. The HRTEM in Figure 2c reveals that the ZnO NW is a single crystalline with [0001] as the preferential growth direction. The marked lattice spacing of 0.52 nm corresponds to the interplane spacing of the (0001) planes of wurtzite-type hexagonal ZnO crystals. The ZnO NWs were well crystallized, with no impurities being detected within the limit of the EDS. The titanium oxide particles that assembled to the NWs were identified to be amorphous, and the stoichiometric ratio of Ti to O as measured by the EDS was about 1:2 (see Figure 2b). The copper peaks come from the sample supporting grid. The interfaces between the ZnO NWs and TiO2 were flat and clearly visible when the electron beam was perpendicular to the NW axes. The elemental maps of Ti, Zn, and O obtained from an a-TiO2-ZnO NW tip are shown in Figure 2d. The composition variations and clear interface between Ti and Zn distributed regions are clearly demonstrated in this axial heterostructure of the a-TiO2-ZnO NW. Figure 3a shows the morphology of a single Zn2TiO4-ZnO heterostructured NW after annealing treatment at 800 °C with duration 2 h. The nanoparticles capped at the ends of ZnO NW have regular shapes. We found that the diameters of all NWs near the interfaces of the nanoparticles shrunk, which has not been observed in the as-prepared samples. Obviously, the annealing process caused reaction between TiO2 and ZnO NWs. The chemical composition of the nanoparticle has been determined by EDS. The stoichiometric ratio of titanium, zinc, and oxide has been identified to be about 2:1:4. By tilting the sample in TEM, various SAED patterns were obtained from the nanoparticle. These patterns and the tilting angles matched well with the cubic spinel Zn2TiO4 single crystal as shown in Figure 3c and d. These two patterns can be indexed as the [112] and [101] zone axis of the cubic spinel (crystalline parameter a ) 0.846 nm, the JCPDS card 25-1164). Figure 3e shows a HRTEM image recorded at the interface of the cubic spinel Zn2TiO4 particle and ZnO NW. The lattice plane spacings measured from this image match fairly well with that of the {111} of the cubic
Figure 2. (a) Low-magnification TEM image of a single a-TiO2-ZnO heterostructured NW and (b) the corresponding EDS recorded on the NW and the nanoparticle at the tip of the NW. (c) HRTEM image recorded at the interface of the a-TiO2-ZnO heterostructure. The lattice plane spacing of 0.52 nm matched the {0001} planes of the wurtzite ZnO structure fairly well. (d) Single a-TiO2-ZnO heterostructured NW and the corresponding EDS elemental mappings of titanium, zinc, and oxygen.
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Figure 3. (a) Low-magnification TEM image of a single Zn2TiO4-ZnO heterostructured NW and (b) the corresponding EDS recorded on the nanoparticle at the tip of the NW. (c) and (d) are SAED patterns taken from the [112] and [101] zone axes of the cubic spinel Zn2TiO4 nanoparticle shown in (a). (e) HRTEM image recorded at the interface of the Zn2TiO4-ZnO heterostructure. The lattice plane spacing of 0.48 nm matched the {111} planes of the cubic spinel Zn2TiO4, and the spacing of 0.26 nm matched the {0001} planes of the wurtzite ZnO structure. (f) Single Zn2TiO4-ZnO heterostructured NW and the EDS elemental mappings of titanium, zinc, and oxygen.
spinel Zn2TiO4 and the {0001} of wurtzite ZnO. The marker plane locates clearly at the interface as indicated by the dashed line. There is no secondary phase and disordered phase at the interface, indicating that the cubic spinel Zn2TiO4 crystal directly grew on the ZnO NW. The growth direction was determined to be along the [111] direction, which was parallel to the growth direction (c-axis) of the ZnO NW. The elemental maps of Ti, Zn, and O of a single Zn2TiO4-ZnO NW are shown in Figure 3f. Apparently, Zn2+ diffuses from the ZnO NW into the a-TiO2 nanoparticle, while the Ti remained at the nanoparticle during reaction. In the meantime, we noticed that the diameter of the NW near the interface of the a-TiO2 nanoparticle tapered off (Figure 3a, f) after reaction, indicating the diffusion of Zn2+ from the surface of ZnO NWs into the nanoparticle. According to our TEM observation, there are two orientation relationships between the cubic spinel Zn2TiO4 particles and ZnO NWs.
(111j)Zn2TiO4 ||(0001)ZnO, (111j)Zn2TiO4 ||(0001)ZnO,
[2j20]Zn2TiO4 ||[112j0]ZnO [2j20]Zn2TiO4 ||[11j00]ZnO
These two orientation relations are illustrated by the HRTEM images and their fast Fourier transform (FFT) patterns in Figure 4. For the growth of heteroepitaxial cubic crystals along the [111] direction, hexagonal substrates with [0001] orientation are often used. This is because of the symmetry matching at the interface in order to minimize the interface energy. The calculated lattice mismatches for these two configurations are -8.75% and 5.82%, respectively. We believe that the small difference of these two mismatches makes these two orientation configurations both favorable and frequently found in our TEM observation. As illustrated in Figure 5a(I)-(III), there are two possible reaction mechanisms for the formation of ZnO-based spinel
crystals, the unilateral diffusion mechanism and the counter diffusion mechanism. To distinguish these two reaction mechanisms, it is necessary to determine the relative position between the marker plane and as-formed cubic spinel Zn2TiO4. The marker planes for our a-TiO2-ZnO heterostructured NWs are located at the interface between a-TiO2 nanoparticles and ZnO NWs. In our previous work, we observed that the amorphousTiO2 nanoparticles attached at the ends of ZnO nanowires transformed into anatase and rutile phases at 300 and 600 °C, respectively, while the TiO2 nanoparticles still located at the ends of ZnO nanowires. 11 There was no observable surface diffusion of TiO2 along the ZnO nanowire surface at relatively low annealing temperatures. As the annealing temperature increased to 650 °C, the TiO2 nanoparticles have already transformed into Zn2TiO4 within 10 min (see Figure 5b-f). A featured morphology change in the annealed products was that the region of the NW close to the interface of the amorphousTiO2 nanoparticle tapered off (Figure 3a and Figure 5a-b). This change indicated the diffusion of Zn2+ and O2- into TiO2 by a solid-solid reaction during the annealing process. The mechanism II of unilateral diffusion of Ti4+ is therefore ruled out. In addition, it was noted that the annealing temperature of 650 °C was low enough to inhibit the possible reconfiguration of Zn2TiO4-ZnO, which might happen at high temperatures (e.g., 800 °C). If mechanism III works, the region of the ZnO nanowire close to the TiO2 nanoparticle will transform into Zn2TiO4 through diffusion of TiO2 into ZnO, and a characteristic “T”-shaped Zn2TiO4 nanoparticle (Figure 5a(II)) could be observable by HRTEM. However, our TEM investigations showed that the regions of ZnO nanowires close to the TiO2 nanoparticles did not transform into Zn2TiO4 and their surfaces were free of TiO2. The EDS elemental mappings for the distribution of oxygen, titanium, and zinc in the heterostructure further supported the unilateral diffusion model in our samples (Figure 5b). Apparently, Zn2+ and O2- diffuse from the ZnO NW into the a-TiO2 nanoparticle, while the titanium remained at the nanoparticle during reaction. The same results were also
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Figure 4. (a) and (e) TEM images of two Zn2TiO4-ZnO heterostructured NWs with different oriented relationships. (b) and (c) are their HRTEM images and corresponding FFT patterns, which are recorded at the particle and nanowire regions in (a), respectively. (f) and (g) are recorded from the heterostructure NW of (e). (d) and (h) are schematic illustrations of the two orientation relationships between the spinel Zn2TiO4 and the wurtzite ZnO.
Figure 5. (a) Schematic illustration of the diffusion mechanisms during the formation of Zn2TiO4. (b) TEM images of a single Zn2TiO4-ZnO heterostructure and the corresponding EDS elemental mappings of titanium, zinc, and oxygen. (c) HRTEM image recorded at the interface of the Zn2TiO4-ZnO heterostructure annealed at 650 °C for 10 min. (d) Enlarged HRTEM image of a Zn2TiO4 head. The lattice plane spacing of 0.48 and 0.29 nm matched the {111} and {110} planes of the cubic spinel Zn2TiO4, respectively. (e) and (f) SAED patterns taken at the nanoparticle and the NW, confirming that the a-TiO2 nanoparticle has transformed to cubic spinel Zn2TiO4.
obtained from the samples annealed at 700 and 800 °C for 10 min. Based on these results, we believe that the formation mechanism of the Zn2TiO4 structure should be explained using mechanism I, a unilateral surface diffusion of ZnO into TiO2. The present novel TiO2/ZnO nanoheterostructures showed unusual diffusion behavior compared to the previous studies.5,6,14 For example, Yang Yang et al.5 and Yang Yi et al.6 reported
the formation of the cubic spinel Zn2TiO4 nanostructures by annealing Ti/TiO2-coated ZnO nanowires. They have suggested that the diffusion speed of Ti is faster than or comparable with those of Zn. Manik et al.14 reported the formation of spinel Zn2TiO4 nanoparticles by high-energy ball milling of mixed ZnO and TiO2 particles at room temperature. Based on the analysis of the lattice strains in ZnO by XRD diffraction, they suggested
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that the Zn atoms in ZnO are preferably substituted by the Ti atoms. The formation of voids at interfaces has been considered as one of the criteria to identify the diffusion mechanism in the phase transformation of ternary oxides from core-shell nanowires. This criterion is only valid when the outer layer is stable during the annealing treatment such as in the ZnO-Al2O3 and ZnO-SiO2 systems. However, this criterion may not be valid for ZnO-TiO2 core-shell nanowires. This is because no interfacial voids were observed in the intermediate or final products due to the instable amorphous TiO2/Ti outer layer which transformed into isolated anatase TiO2 at a temperature above 300 °C.5,6 Therefore, it was hard to figure out the diffusion behavior of Ti and Zn during the formation of Zn2TiO4 from the ZnO-TiO2 core shell heterostructures. From this point, our TiO2-ZnO nanowire axial heterostructures provide an alternative kind of coupled structure suitable for the studies of formation mechanisms of spinel type ternary compounds. Our observation of unilateral diffusion of ZnO into a-TiO2 also provided crucial information for understanding the formation of ZnO-based ternary compound nanotubes and NWs, explaining the growth phenomena and designing new nanostructures. Fan et al.3 suggested that the hollow spinel nanotubes can be fabricated through the formation of Kirkendall voids by annealing the core-shell metal oxide NWs when the out-diffusion of the core material through the spinel is faster than the indiffusion of the shell. This approach has been successfully used to fabricate spinel ZnAl2O4 and Zn2SiO4 nanotubes.2 However, the annealing of core-shell Ti/TiO2-ZnO NWs did not result in spinel Zn2TiO4 nanotubes but rather spinel Zn2TiO4 NWs and Zn2TiO4-ZnO core-shell NWs owing to the instable amorphous TiO2/Ti outer layers.5,6 Therein, both the mutual diffusion rates of core and the structural stability of the outer layer materials determine the final morphologies of the annealed core-shell nanoheterostructures.
Cheng et al. 4. Conclusion In summary, we synthesized a novel Zn2TiO4-ZnO NW heterostructure by a two-stage fabrication process. The Zn2TiO4 heads are cubic spinel single crystalline with two different orientation relationships with the ZnO NWs. Structural and elemental analyses revealed a unilateral surface diffusion of ZnO into TiO2 which played an important role in the formation of the Zn2TiO4 structure. The growth mechanism and the technique developed in this paper provide an essential guidance for rational design and fabrication of ZnO-based ternary heterostructures. Acknowledgment. This work was financially supported by the Research Grants Council of Hong Kong (Project Nos. CityU5/CRF/08, 603408, and 604009). References and Notes (1) Wang, Z. L. Appl. Phys.: Mater. Sci. Proc. 2007, 88, 7. (2) Fan, H. J.; Yang, Y.; Zacharias, M. J. Mater. Chem. 2009, 19, 885. (3) Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharias, M.; Gosele, U. Nat. Mater. 2006, 5, 627. (4) Feng, P.; Zhang, J. Y.; Wan, Q.; Wang, T. H. J. Appl. Phys. 2007, 102, 074309. (5) Yang, Y.; Scholz, R.; Fan, H. J.; Hesse, D.; Gosele, U.; Zacharias, M. ACS Nano 2009, 3, 555. (6) Yang, Y.; Sun, X. W.; Tay, B. K.; Wang, J. X.; Dong, Z. L.; Fan, H. M. AdV. Mater. 2007, 19, 1839. (7) Branson, D. L. J. Am. Ceram. Soc. 1965, 48, 591–595. (8) Rigby, E. B.; Cutler, I. B. J. Am. Ceram. Soc. 1965, 48, 95–99. (9) Fan, H. J.; Knez, M.; Scholz, R.; Hesse, D.; Nielsch, K.; Zacharias, M.; Gosele, U. Nano Lett. 2007, 7, 993. (10) Pineda, M.; Fierro, J. L. G.; Palacios, J. M.; Cilleruelo, C.; Garcia, E.; Ibarra, J. V. Appl. Surf. Sci. 1997, 119, 1. (11) Cheng, C.; Yu, K. F.; Cai, Y.; Fung, K. K.; Wang, N. J. Phys. Chem. C 2007, 111, 16712. (12) Cheng, C.; Lei, M.; Feng, L.; Wong, T. L.; Ho, K. M.; Fung, K. K.; Loy, M. M. T.; Yu, D. P.; Wang, N. ACS Nano 2009, 3, 53. (13) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. (14) Manik, S. K.; Bose, P.; Pradhan, S. K. Mater. Chem. Phys. 2003, 82, 837.
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