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Brush-Like Hierarchical ZnO Nanostructures: Synthesis, Photoluminescence and Gas Sensor Properties Yuan Zhang,†,‡ Jiaqiang Xu,*,†,§ Qun Xiang,† Hui Li,†,‡ Qingyi Pan,† and Pengcheng Xu§ Department of Chemistry, College of Science, Shanghai UniVersity, Shanghai 200444, China, Department of Physics, College of Science, Shanghai UniVersity, Shanghai 200444, China, and State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China ReceiVed: September 12, 2008; ReVised Manuscript ReceiVed: December 28, 2008
Brush-like hierarchical ZnO nanostructures assembled from initial 1D ZnO nanostructures were prepared from sequential nucleation and growth following a hydrothermal process. The morphology, structure, and optical property of hierarchical ZnO nanostructures were characterized by X-ray diffraction (XRD), fieldemission scanning electron microscopy (FE-SEM), and photoluminescence (PL) studies. The FE-SEM images showed that the brush-like hierarchical ZnO nanostructures are composed of 6-fold nanorod-arrays grown on the side surface of core nanowires. Compared with ZnO nanowires, brush-like hierarchical ZnO nanostructures easily fabricated satisfactory ethanol sensors. The main advantages of these sensors are featured in excellent selectivity, fast response (less than 10 s), high response (sensitivity), and low detection limit (with detectable ethanol concentration in ppm). 1. Introduction Nanoscale materials have stimulated great interest in current materials science due to their importance in such basic scientific researches and potential technology applications as microelectronic devices,1 chemical and biological sensors,2 light-emitting displays,3 catalysis,4 and energy conversion and storage devices.5 Previous studies indicated that the shape of nanoscale materials have a profound influence on their properties.6 This has led to intensive investigation on quantum dots,7 nanowires,8 nanotubes,9 and self-assembled monolayers (SAMs).10 In recent years, much attention has been paid to three-dimensional and hierarchical architectures that derive from the above-mentioned low dimensional nanostructures as building blocks, due to various novel applications. For example, the Lieber group has reported that core/shell coaxial silicon nanowires architectures can be employed as solar cells.1 Wang reported that a novel hierarchical nanostructure based on Kevlar fibers coated with ZnO nanowires could serve as nanogenerators.11 Though tremendous progress has been made in this significant field, there are still great demands on the synthesis of alternative threedimensional and hierarchical architectures with novel or potential applications. Hierarchical ZnO architectures have been extensively produced with gas-phase approaches.12-14 With these synthetic methods, nanowire arrays,15 nanohelitics,16 nanopropeller,17 and tower-like nanocolumns18 have been successfully prepared. However, these protocols often require high temperature and induce impurities in the final products when catalysts and templates are introduced to the reaction system. In practice, this made it difficult to obtain organic/inorganic hybrid hierarchical architectures. In addition, although the solution-based synthetic strategies are simple and effective in the production of the * Corresponding author. Tel: +86 21 66134728. Fax: +86 21 66134725. E-mail:
[email protected]. † Department of Chemistry, Shanghai University. ‡ Department of Physics, Shanghai University. § Chinese Academy of Sciences.
building blocks of hierarchical nanostructures such as nanoparticles,19 nanowires,20 and nanorods,21 achievement of hierarchical architectures from such techniques remains a challenge. Herein, we developed a simple nucleation and growth strategy to synthesize brush-like hierarchical ZnO nanostructures. The two step seeded-growth approach allows stepwise control and optimization of experimental conditions and provides an opportunity for rational design and synthesis of controlled architectures in nanostructures.22-24 We also investigated the effect of morphology and structure of brush-like hierarchical ZnO nanostructures on its gas sensing responses. The results show that ZnO hierarchical nanostructures display better ethanol sensing property than that of ZnO nanowires. 2. Experimental Section All reagents employed were analytically pure and used as received from Shanghai Chemical Industrial Co. Ltd. (Shanghai, China) unless otherwise mentioned. 2.1. Synthesis. The strategies to fabricate the brush-like hierarchical ZnO nanostructures are summarized as follows. First, the ZnO nanowires with a uniform shape were synthesized as described elsewhere,25 which was used as the initial material to grow hierarchical ZnO nanostructures. Subsequently, a saturated solution of Zn(OH)42- was prepared by dissolving excess ZnO in 10 mL of NaOH solution (5 mol/L) for growing hierarchical ZnO nanostructures. Third, the ZnO nanowires (0.05 g) were uniformly suspended in deionized water (37 mL) in an ultrasonic bath. The suspension was mixed with the Zn(OH)42saturated solution (3 mL). After the mixture was transferred into a Teflon-lined autoclave (50 mL), it was kept at 100 °C for 10 h. Finally, upon the hydrothermal treatment a white precipitate was formed, which was washed thoroughly with ethanol and distilled water in sequence, and dried at 60 °C under vacuum for 6 h. 2.2. Characterization. The initial ZnO seeds were characterized by transmission electron microscopy (TEM, JEM-200CX, using an accelerating voltage of 160 kV). The morphology of
10.1021/jp8092258 CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
Brush-Like Hierarchical ZnO Nanostructures
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Figure 3. XRD pattern of the brush-like hierarchical ZnO nanostructures. Figure 1. Sketch of the gas sensor.
Figure 2. Measuring electric circuit of the gas sensor.
hierarchical ZnO nanostructures was observed by field emission scanning electron microscopy (FE-SEM, JSM-6700F). The crystal phase of as-synthesized products was identified by powder X-ray diffraction (XRD) analysis using a D/max 2550 V diffractometer with Cu KR radiation (λ ) 1.54056 Å) (Rigaku, Tokyo, Japan), and the XRD data were collected at a scanning rate of 0.02 deg s-1 for 2θ in a range from 10° to 70°. 2.3. Photoluminescence (PL) Measurement. Room temperature PL measurements were performed on a Hitachi RF5301PC spectrofluophotometer using the 350 nm Xe laser line as the excitation source. 2.4. Gas Sensor Fabrication and Response Test. The ZnO powder was mixed with Terpineol and ground in an agate mortar to form a paste. The resulting paste was coated on an alumina tube-like substrate on which a pair of Au electrodes had been printed previously, followed by drying at 100 °C for about 2 h and subsequent annealing at 600 °C for about 2 h. Finally, a small Ni-Cr alloy coil was inserted into the tube as a heater, which provided the working temperature of the gas sensor. The schematic drawing of the as-fabricated gas sensor is shown in Figure 1. In order to improve the long-term stability, the sensors were kept at the working temperature for several days. A stationary state gas distribution method was used for testing gas response (Air humidity: 47%). In the measurement of electric circuit for gas sensors (Figure 2), a load resistor (Load resistor value: 470 kΩ) was connected in series with a gas sensor. The circuit voltage was set at 10 V, and the output voltage (Vout) was the terminal voltage of the load resistor. The working temperature of a sensor was adjusted through varying the heating voltage. The resistance of a sensor in air or test gas was measured by monitoring Vout. The test was operated in a measuring system of HW-30A (Hanwei Electronics Co. Ltd., P.R. China). Detecting gases, such as C2H5OH, were injected into a test chamber and mixed with air. The gas response of the sensor in this paper was defined as S ) Ra/Rg (reducing gases) or S ) Rg/Ra (oxidizing gases), where Ra and Rg were the resistance in air and test gas, respectively. The response or recovery time was expressed as the time taken for the sensor output to reach 90% of its saturation after applying or switching off the gas in a step function.
Figure 4. FE-SEM images of the brush-like hierarchical ZnO nanostructures: (a) at low magnification and (b) at medium magnification.
3. Results and Discussion 3.1. Structure and Morphology. In the XRD pattern of the as-synthesized products (Figure 3), all of the peaks were well indexed to hexagonal wurtzite ZnO (JCPDS No. 36-1451, a ) 0.3249 nm, c ) 0.5205 nm) with high crystallization. No characteristic peaks were observed for impurities. The FE-SEM images of hierarchical ZnO nanostructures (Figure 4) were observed at low and medium magnifications, respectively. From the low magnification image (Figure 4a), the secondary nanorods self-organized into very regular arrays, which mimic brush-like hierarchical nanostructures. These nanorod arrays that grew onto one common central nucleus could also be revealed from the protrudent brush-like structures (Figure 4a). The medium magnification (Figure 4b) clearly indicated that these secondary nanorod arrays were brush-like 6-fold symmetry. More careful
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Figure 5. TEM image of the ZnO nanowire seeds.
observation of the morphology of hierarchical nanostructures indicated that the nanorod arrays grew on the side surface of core nanowire. The reason for nanorod arrays to grow into 6-fold symmetry may arise from the hexagonal symmetry of the major core.26 The central stems provide its six prismatic crystal planes/ facets as growth platforms for branching of multipod units. The synthesis of 6-fold ZnO nanostructures have been reported;17,26-28 however, the solution-based approach to this structure is rarely disclosed. Under hydrothermal conditions, heteronucleation can take place, and the interfacial energy between crystal nuclei and substrates is usually smaller than that between crystal nuclei and solutions.22 Therefore, the secondary rod-like branches can grow on the wire-like ZnO central core. 3.2. Impact of the Reaction Conditions on the Growth of ZnO Hierarchical Nanostructures. Our studies suggest that the morphology of ZnO seeds and the concentration of OHion in the reaction system are the key factors for the formation of ZnO hierarchical nanostructures. First, ZnO nanostructures with different morphology were used as seeds for the nucleation and growth process. In the synthesis of brush-like ZnO hierarchical nanostructures, high aspect ratio (height/width) ZnO nanowires were used as seeds. These ZnO nanowires have typical diameters of 80-100 nm and lengths up to 10 µm (Figure 5). In order to study the effect of the seeds morphology on the growth of brush-like ZnO hierarchical nanostructures, a lower aspect ratio of ZnO nanorods (200 nm diameter, aspect ratio
