Catalyst-free direct vapor-phase growth of Zn1−xCu x O micro-cross structures and their optical properties
© Xu et al; licensee Springer. 2013
Received: 26 December 2012
Accepted: 15 January 2013
Published: 22 January 2013
We report a simple catalyst-free vapor-phase method to fabricate Zn1−xCu x O micro-cross structures. Through a series of controlled experiments by changing the location of the substrate and reaction time, we have realized the continuous evolution of product morphology from nanorods into brush-like structures and micro-cross structures at different positions, together with the epitaxial growth of branched nanorods from the central stem with the time extended. The growth mechanism of the Zn1−xCu x O micro-cross structures has been proposed to involve the synthesis of Cu/Zn square-like core, surface oxidation, and the secondary growth of nanorod arrays. By the detailed structural analysis of the yielded Zn1−xCu x O samples at different locations, we have shown that the CuO phases were gradually formed in Zn1−xCu x O, which is significant to induce the usual ZnO hexagonal structures changing into four-folded symmetrical hierarchical micro-cross structures. Furthermore, the visible luminescence can be greatly enhanced by the introduction of Cu, and the observed inhomogeneous cathode luminescence in an individual micro-cross structure is caused by the different distributions of Cu.
KeywordsCu-doped ZnO Micro-cross structures Optical properties Epitaxial growth Catalyst-free vapor-phase method
One-dimensional (1D) ZnO nanostructures (e.g., nanowires, nanorods, and nanotubes) are promising with extensive applications in nanoelectronics and nanophotonics due to their efficient transport of electrons and excitons . In recent years, increasing attention has been paid to three-dimensional (3D) hierarchical ZnO architectures which derived from 1D nanostructures as building blocks based on various novel applications [2–6]. To date, different kinds of hierarchical branched ZnO nanostructures, including nanobridges , nanoflowers [2, 8], rotor-like structures , and nanotubes surrounded by well-ordered nanorod structures , have been reported by using either solution-phase or vapor-phase method. However, these processes often require high temperature, complex multi-step process, or introduction of impurities by the templates or foreign catalysts in the reaction system. Therefore, it is still a challenge to find a simple and controllable synthetic process to fabricate 3D hierarchical ZnO architectures with novel or potential applications.
On the other hand, doping is a widely used method to improve the electrical and optical properties of semiconductors . Copper, considered as a valuable dopant for the achievement of long-searched-for p-type ZnO , can serve not only as a luminescence activator but also as a compensator of ZnO . In addition, Cu doping, leading to form donor-acceptor complexes, can induce a polaron-type ferromagnetic order in ZnO [14, 15]. Zn1−xCu x O has been previously employed as phosphor , an active material in varistors  and spintronic devices . Up to now, most of the investigations in the Zn1−xCu x O system have been focused on thin films and 1D nanostructures, such as Cu-doped ZnO nanowires , nanonails, and nanoneedles . 3D hierarchical Zn1−xCu x O nanostructures, posing many unique properties arisen from their special geometrical shapes and inherently large surface-to-volume ratios, show considerable promise for the development of nanodevices with multiple functions (e.g., gas sensor  and photocatalytic hydrogen generation ). However, thus far, there have been no reports of such Zn1−xCu x O hierarchical nanostructures.
Herein, we realize a simple catalyst-free vapor-phase deposition method to synthesize the Zn1−xCu x O hierarchical micro-cross structures. The branched nanorods are neatly aligned on four sides of the backbone prism, assembling the shape of crosses. The subtle variations of environmental conditions have triggered the observed continuous morphological evolution from 1D nanorod to 3D hierarchical micro-cross structures. A possible growth mechanism for the micro-crosses has been proposed. Detailed structural and optical studies reveal that the CuO phases are gradually formed in Zn1−xCu x O and Cu concentration can greatly influence the structural defects. Interestingly, the Zn1−xCu x O micro-cross structure exhibits distinct inhomogeneous cathode luminescence (CL), which can be attributed to the different defect concentrations induced by Cu through characterizing the emission of defects and contents of Cu over the individual micro-cross structure.
The morphology and microstructure of the structures were characterized by field-emission scanning electron microscopy (FE-SEM; Philips XL30FEG, Portland, OR, USA) with an accelerating voltage of 5 kV, high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100 F, Akishima-shi, Japan), and X-ray diffraction (XRD; Bruker/D8 Discover diffractometer with GADDS, Madison, WI, USA) equipped with a Cu Kα source (λ = 1.5406 Å). Energy-dispersive X-ray (EDX) analysis was also performed during the FE-SEM observation. The bonding characteristics were analyzed by PHI Quantum 2000 X-ray photoelectron spectroscopy (XPS; Chanhassen, MN, USA). The micro-Raman in the backscattering geometry and photoluminescence (PL) spectra were recorded at room temperature using a Jobin Yvon LabRAM HR800UV micro-Raman system (Kyoto, Japan) under Ar+ (514.5 nm) and He-Cd (325.0 nm) laser excitation, respectively. The CL measurements were carried out at room temperature using a Gatan Mono-CL system-attached FE-SEM (Pleasanton, CA, USA) with the accelerating voltage of 10 kV.
Results and discussions
As a reference, specimens of pure ZnO nanostructures were grown in the tube furnace system using Zn powder as the only source material. We can observe that the as-grown products always present the commonly reported nanowire morphology (Figure 1b). The length of the undoped nanowires ranges from 4 to 8 μm, and the diameter is about 150 nm. The high-magnification SEM image is shown in Figure 1 (b’), demonstrating uniform hexagonal cross sections and a smooth surface. With the introduction of Cu in the precursor, the as-grown Zn1−xCu x O samples exhibit three different morphologies (see in Figure 1c,d,e), which are deposited on the substrates at different positions (marked as C, B, and A in Figure 1a, respectively). For the sample at position C (as shown in Figure 1c), the nanorods are formed, of which the lengths become shorter (approximately 1.5 μm) and the diameters become bigger (approximately 250 nm). Some Zn1−xCu x O nanorods display deformed hexagon sections (see Figure 1 (c’)), which may be induced by doping. As seen in Figure 1d, a kind of brush-like structures appears (at position B). These brushes are randomly assembled by the nanowires. For the sample at position A, the low-magnification SEM image in Figure 1e shows that a large quantity of micro-cross structures formed. The definition of micro-cross comes from the geometrical similarity to the cross structures.
Further morphological and structural analysis of the micro-cross structure can be characterized by the HRTEM and selected-area electron diffraction (SAED) techniques. Figure 2e presents the TEM morphology of the individual cross structure, which consists of the nanorod in the central stem, together with the nanorod arrays on the side surface of the core. The central stem is too thick to be detected from the TEM observation. The lattice fringes and the corresponding SAED pattern of the cross-like structure are shown in Figure 2f,g, respectively, which are indicated in Figure 2e with a red square. The lattice spacing of 0.52 nm corresponds to the spacing of  crystal planes of wurtzite ZnO.
The above experimental observation reveals that the location of the substrate and reaction time exercise great influences on the morphologies of the products. After Cu is introduced, the ordinary pure ZnO nanowires change into three different morphologies with the variation of the location, i.e., stumpy nanorods, randomly assembled brushes, and well-organized micro-cross structures. It is speculated that the higher temperature (at position A, which is close to the central zone of the tube) is helpful to form a central core of the hierarchical structure. We could find out the clue from the original square-like core, which is shaped in the early stage of the growth process at position A (see Figure 2c). With the reaction time extended, branched nanorods grow epitaxially on the side face of the central stem (see Figure 2a,b). Since Cu has a high-symmetry cubic structure , we can assume that the reason for growing into four-fold hierarchical cross-like structures is because of the tetragonal-symmetry major core induced by the introduction of abundant Cu. In combination with previous reports [24, 25] and the details in our experiment, we suggest the following possible growth mechanism of the Zn1−xCu x O micro-cross structures.
At the stage of temperature rise, oxygen was still not introduced into the tube. Zn/Cu vapor easily condensed into a square-like core on the substrate. When the temperature reached up to the desired 750°C, the core was oxidized with the introduction of oxygen. The cubic core prism could provide its four prismatic facets as growth platforms for the secondary branched nanorod arrays. With the successive arrival of Zn/Cu and O2, the branched nanorods began to grow perpendicular to the central stem. Due to the considerable anisotropy in the speed of the crystal growth along different directions of ZnO, the nanorods with the right orientation, i.e., with the  direction perpendicular to the surface of the prism, could grow much faster than others. The lengths of the branched nanorods are increased with the growth time extended (see Figure 2a,b). In the whole growth process, there are no external metallic catalysts (e.g., Au and In) involved in the formation of micro-cross structures. That is, the 3D hierarchical micro-cross structure is synthesized by a simple catalyst-free direct vapor-phase growth method.
The structural phase evolution of the as-fabricated products with different Cu concentrations was also investigated by XRD, which is shown in Figure 3b. It is clear that all the diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO (JCPDS No. 36–1451) in the undoped one. In contrast, five small new phases emerge in the sample with the Cu content of 7%. These new phases in the XRD spectrum correspond to CuO (matched with JCPDS No. 01–1117), owing to the fact that the solubility of Cu ions in ZnO is quite low . Moreover, it is noted that with the increase of Cu content, these CuO diffraction peaks become more obvious and stronger. Meanwhile, the ZnO diffraction peaks remain nearly unshifted, indicating that the added Cu elements have no effects on the crystal structure of ZnO, which is coincident with the HRTEM results in Figure 2f.
On the other hand, three additional modes at around 290, 340, and 628 cm−1 can be observed. They are attributed to the Ag, B1g, and B2g modes of CuO due to the vibrations of oxygen atoms, respectively [33, 34]. From Figure 5, it is obvious that the intensity of the CuO peaks enhanced while that of ZnO peaks decreases with the Cu concentration increases up to 33%. Such behavior is caused by the competition of Zn and Cu during the oxidization process. In the sample with the highest Cu content of 33%, the formation of CuO is dominant, in spite of the fact that the lower melting point and higher vapor pressure of Zn than those of Cu under the same conditions . The formation of CuO is significant to induce the usual ZnO hexagonal structures changing into four-folded cross-like structures, in good agreement with the growth mechanism we have proposed above.
As can be clearly observed from Figure 6, the undoped ZnO possesses a strong near-band-edge UV emission together with a weak visible emission, indicating that the undoped ZnO nanostructures have a fairly high quality with low defect concentration (its PL intensity was 10 times magnified). After Cu is introduced, the UV emission is rapidly suppressed while the visible luminescence is greatly enhanced compared with the undoped counterpart, suggesting the poorer crystallinity and greater level of structural defects introduced by Cu ion incorporation into ZnO. The intensity ratio of the visible band emission to the UV peak increases from approximately 0.2 to approximately 150 with the Cu content change from 0% to 33%, demonstrating that the Cu doping strongly increases the concentration of defects. Nevertheless, the defects are believed to significantly improve a variety of surface properties, such as heterogeneous catalysis, corrosion inhibition, and gas sensing, which have been addressed by theoretical calculation and experimental data [38–40]. Furthermore, we have also presented in the inset the enlarged view of the UV peak between 360 and 405 nm. It is obvious that the introduction of Cu will cause a little redshift of the UV peak (34 meV under Cu contents from 0% to 33%) compared with the undoped one, i.e., a reduction of ZnO bandgap caused by the Cu doping.
Figure 7c illustrates the typical CL spectra, which are acquired at the center stem (noted as ‘0’ on the axis in Figure 7b) and four different locations along one branched nanorod. The spectra exhibit similar features as the PL spectra, that is, a comparatively weak UV peak due to the NBE emission and a broad, strong peak in the visible region, which is attributed to the deep-level (DL) emission affiliated with defects and impurities. To further reveal the variation of the defect concentration, the intensity ratios of the DL emission to the NBE emission (IDL/INBE) at different locations are plotted in Figure 7d (marked as ‘CL Ratio’). We can notice that the ratio of IDL/INBE decreases from approximately 92 to approximately 5 with the location change from 0 to 1,000 nm, demonstrating that the concentration of defects strongly depends on the location. The center part of the cross-like structure exhibits the highest defect density. We have also performed the EDX analysis on three different location points along the branched nanorod to illustrate the evolution of the Cu content (marked as ‘Cu Content’ in Figure 7d). It is clear that the central zone of the cross structure has the higher Cu concentration of approximately 53.6%, while the edge part of the branched nanorod has ultra-low Cu content (nearly zero). The introduction of abundant Cu in the core has induced the usual ZnO hexagonal structures changing into four-folded symmetrical micro-cross structures, which is consistent with the abovementioned growth mechanism and EDX analysis (shown in Figure 2d). The Cu contents are consistently and significantly reduced from the central zone to the edge part of the branched nanorod, which may be caused by the Cu diffusion at the stage of epitaxial growth of branched nanorods from the central core. The spatial differences of the Cu content along the structure would induce the variation of the defect distribution, resulting in the distinct inhomogeneous luminescence within one micro-cross structure.
In summary, we report a new and delicate cross-like Zn1−xCu x O structure, in which four-sided branched nanorod arrays grow perpendicular to the side surfaces of the central stem. This structure is formed through the direct vapor-phase deposition method but without introducing any catalyst. By changing the reaction time, the possible growth mechanism of the micro-cross structures has been proposed to involve the synthesis of Cu/Zn core, surface oxidation, and the secondary growth of the branched nanorods. The location of the substrate is an important factor determining the morphologies (from 1D nanorods to 3D micro-cross structures) and Cu concentrations (from 7% to 33%) of the yielded Zn1−xCu x O samples. We have employed the XRD, Raman, and PL spectroscopies to demonstrate that the formation of CuO-related phases and concentration of the defects in the products have been greatly influenced by the Cu content. Moreover, inhomogeneous CL has been observed in a single micro-cross structure, which is generated from structural defects created by the Cu incorporation into ZnO. The presented method is expected to be employed in a broad range to fabricate other similar metal-doped ZnO 3D hierarchical structures for their potential device applications.
A1 transverse optical
E1 longitudinal optical
High-frequency branch of the E2
Field-emission scanning electron microscopy
High-resolution transmission electron microscopy
Selected-area electron diffraction
X-ray photoelectron spectroscopy
This work was supported by the National Major Basic Research Project (2012CB934302) and the Natural Science Foundation of China (11174202 and 61234005).
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