Large-scale uniform ZnO dumbbells and ZnO/ZnS hollow nanocages were successfully synthesized via a facile hydrothermal route combined with subsequent etching treatment. The nanocages were formed through preferential dissolution of the twinned (0001) plane of ZnO dumbbells. Due to their special morphology, the hollow nanocages show better sensing properties to ethanol than ZnO dumbbells. The gain in sensitivity is attributed to both the interface between ZnO and ZnS heterostructure and their hollow architecture that promotes analyte diffusion and increases the available active surface area.
KeywordsZnO/ZnS Hollow Nanocages Ethanol sensing Interface
Nanomaterials with hollow appearance are drawing intense research interest because these structures often exhibit interesting properties that are different from those of particles, thus making them attractive from both scientific and technological viewpoints. The fabrication of hollow structures with homogenous shells will open up possibilities for various application fields, such as controlled-release capsules, artificial cells for drug delivery, lightweight fillers, shape-selective adsorbents, and catalysts . Several methodologies have been developed to achieve these special nanostructures, for example galvanic-replacement reactions have been successfully used to produce Au nanoboxes , nanocages ; single-crystalline Pd nanoboxes, and nanocages were obtained by a surface metal-corrosion process , and Co cubic nanoskeletons have been synthesized by a simple one-pot solution method . Furthermore, hetero-nanostructures with hollow appearance have attracted considerable attention nowadays because of their amazingly complicated structures as well as their outstanding properties and broad potential applications [6–8]. A number of remarkable features of these materials include huge specific area, flexible chemical compositions, and multiphase anisotropic interfaces . Most of these properties are desired pursuits of scientists.
As important wide band gap semiconductors, ZnO and ZnS have a wide range of applications for optical and electric devices. And the studies of ZnO/ZnS heterostructures with various morphologies, such as nanorings , biaxial nanowires , and saw-like nanostructures , have been reported. However, the synthesis of ZnO/ZnS heterojunction hollow nanocages still remains a challenge. In this work, a template-free hydrothermal route combined with subsequent etching treatment is demonstrated for the synthesis of ZnO/ZnS hollow nanocages. A comparative gas-sensing study between the as-prepared ZnO dumbbells and ZnO/ZnS hollow nanocages was performed to depict the superior sensing properties of these hollow hetero-nanocages.
Preparation of ZnO Dumbbell Nanostructure
In a typical preparation process, the hexamethylenetetramine (HMT) aqueous solution (10 mmol L−1) was mixed with zinc nitrate (Zn(NO3)2) aqueous solution (10 mmol L−1) under vigorous magnetic stirring at 90°C for 3 h and then cooled to room temperature naturally. The resultant white solid product was centrifuged, washed with distilled water and alcohol in turns, and then dried in vacuum at 60°C for 6 h.
Preparation of ZnO/ZnS Hollow Nanocages
To convert ZnO to ZnO/ZnS hetero-nanostructure, thioacetamide (CH3CSNH2) was used to supply sulfide ions. CH3CSNH2 aqueous solution (4 mM) was added into 50 mL as-prepared ZnO suspension. Then, the mixture solution was heated under stirring on a hot plate at 90°C for 3 h. After the reaction, the obtained ZnO/ZnS precipitates were separated by centrifugation, washed with deionized water, and dried at 60°C for 2 h.
The X-ray diffractometry (XRD) for the crystal structure of the products was carried out in a Rigaku D/max 2500v/pc diffractometer. The morphology of samples was observed using a Hitachi S-4800 field-emission scanning electron microscope (FESEM) and a FEI Tecnai G2 F20 transmission electron microscope (TEM) with a field-emission gun operating at 200 kV. Photoluminescence (PL) analyses with a Hitachi F-4500 fluorescence spectrometer were performed on aqueous solutions directly at room temperature using 325 nm as the excitation wavelength. The gas-sensitive properties were measured using a static test system (Hanwei Electronics, China).
Results and Discussion
Such reactions can proceed preferentially at the end planes and ridges of the ZnO prism, where many defects were observed in SEM and TEM images, as a result, ZnS nanocrystals can be formed at those sites and connect into a frame; the further growth of ZnS nanocrystals would consume ZnO at the planar surface and the inner of the dumbbells, which results in nanocages with ZnS shell and regular interior space.
The enhanced sensitivity of the ZnO/ZnS hollow nanocages sensor to ethanol gas is attributed to both the unique morphology and the heterojunction between ZnO and ZnS. Theoretically, the rate of the surface reaction is in proportion to the number of available adsorption sites on the outer surface of nanostructure . This illustrates that the loose and porous structure is clearly more favorable for the diffusion of gas molecules than the compact structure of nanoparticles. Thus, the hollow morphology endows the ZnO/ZnS nanocages strong capability to absorb and desorb gas molecules, and then contributes to the excellent sensing properties. Apart from this, the heterojunction structure between ZnO and ZnS also plays an important role in improving the sensing properties. According to the space-charge layer mode , as the sensor is exposed to air, oxygen molecules will be adsorbed on the oxide surface, and extract electrons in the bulk, leading to a narrow conduction channel. In contrast, when the sensor is exposed to reducing gases, the trapped electrons will be released owing to reactions between reducing gases and adsorbed oxygen molecules, and then the conduction channel changes wider. In ZnO/ZnS heterostructure, oxygen molecules in air should deplete electrons in both ZnS shell and ZnO core . On the other hand, electrons may be scattered by the interface between ZnO and ZnS, which has been proved existence according to the PL spectrum in Fig. 4. The above two factors jointly cause a higher electric resistance of ZnO/ZnS heterostructure in air. As exposed to ethanol vapor, the ethanol reacts with the adsorbed oxygen species, and the extracted electrons from both ZnO core and ZnS shell can be released, resulting in an improved sensitivity of the ZnO/ZnS heterostructure compared with that of bare ZnO.
In summary, based on a hydrothermal route combined with subsequent etching process, ZnO dumbbells and ZnO/ZnS hollow nanocages were synthesized at a large scale. The photoluminescence and gas-sensing properties of these structures were studied. Due to their special morphology, the ZnO/ZnS hollow nanocages show excellent sensing properties to ethanol, including high sensitivity, rapid response rate, and fast recovery. Apparently, the gain in sensitivity is attributed to both the hollow architecture and the heterojunction between ZnO and ZnS nanostructure. Therefore, the prepared ZnO/ZnS hollow nanocages can be used as an efficient sensor for detecting ethanol with fast response and good reproducibility.
This work was supported by the Natural Science Foundation of China (No. 10732020 and No. 50672065), National High-tech R&D Program of China (No. 2007AA021808 and 2009AA03Z301) and the National Science Foundation for Post-doctoral Scientists of China (20090450089).
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