Ultra-fast Microwave Synthesis of ZnO Nanowires and their Dynamic Response Toward Hydrogen Gas
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Ultra-fast and large-quantity (grams) synthesis of one-dimensional ZnO nanowires has been carried out by a novel microwave-assisted method. High purity Zinc (Zn) metal was used as source material and placed on microwave absorber. The evaporation/oxidation process occurs under exposure to microwave in less than 100 s. Field effect scanning electron microscopy analysis reveals the formation of high aspect-ratio and high density ZnO nanowires with diameter ranging from 70 to 80 nm. Comprehensive structural analysis showed that these ZnO nanowires are single crystal in nature with excellent crystal quality. The gas sensor made of these ZnO nanowires exhibited excellent sensitivity, fast response, and good reproducibility. Furthermore, the method can be extended for the synthesis of other oxide nanowires that will be the building block of future nanoscale devices.
KeywordsZnO Microwave synthesis Nanowires FESEM TEM XPES H2gas sensor
Fabrication of nanowires has received remarkable attention as these one dimensional (1D) nanostructures provide an ideal system to investigate the dependence of transport properties on size confinement . Nanowires/nanorods are also expected to play an important role as active components or interconnects in fabricating nanoscale electronics and optoelectronics [2–6]. Zinc oxide (ZnO), a wide band-gap (3.37 eV) semiconductor, is a potentially important material. The naturally high surface-to-volume ratio of quasi 1D ZnO nanowires has made it a contender for chemical and biological sensors. In order to explore these applications, availability in large quantities is necessary. In this regard, various synthesis methods have been explored to fabricate ZnO nanowires, most of which are based on physical and chemical techniques; such as chemical vapor transport and condensation processes, metal-organic chemical vapor deposition, anodic alumina membrane templates, aqueous solution process, nonhydrolytic sol–gel processes, pulsed laser deposition, etc. [7–13]. All these methods mentioned above, however, have the disadvantages of low productivity or severe impurities from their employed assistant, so called catalyst or precursor, which bring about discomfort for their real nanodevice applications. Another limitation is the high production cost due to the complex equipment, long processing time and low growth rate. There is still an underlying question of how to scale-up nanoscale production using these approaches. In this regard, microwave heating is relatively new technique for large-scale nanowire processing which is different from existing conventional process.
Hydrogen is a hopeful potential fuel for cars, buses, and other vehicles and can be transformed into electricity in fuel cells. It is also used in medicine and space exploration as well as in the production of industrial chemicals and food products. Safety is an important issue when using the hydrogen. An explosive mixture can form if hydrogen leaks into the air from a tank or valve, posing a hazard to drivers, equipment operators, or others nearby. The present technology to detect hydrogen has numerous drawbacks which include limited dynamic range, poor reproducibility and reversibility, high power consumption and slow response, etc. Therefore, there is a need to develop new generation of metal oxide-based hydrogen gas sensors with improved performance.
In this work, we present the hydrogen gas sensing properties of ZnO nanowires prepared by a novel one-step ultra-fast microwave assisted method. The results show that the ZnO nanowire gas sensor has reversible response to H2gas. The work demonstrates the possibility of developing ZnO-based low-power consumption gas sensors and extending their applications.
Results and Discussion
Growth Mechanism for the Formation of ZnO Nanowires
Zinc oxide nanowires were grown with the uniform diameter by ultrafast microwave synthesis technique. For the formation of ZnO nanowires Zn metallic particles was used as a source material. Two important factors are responsible for the growth of ZnO nanowires: the formation of crystalline nuclei and axial growth of ZnO nuclei . The formation of nuclei depends on experimental parameters. We used swift microwave synthesis to grow 1D ZnO nanowires. The Zn particles were easily oxidized into ZnO when temperature surpasses to 419 °C. Owing to the fast oxidation, nanosized crystal nuclei were generated. These crystal nuclei were possibly generating sites for ZnO vapors, and thus the nanowires were most likely grown under the control of ZnO crystal growth habit. With the increase of reaction time and temperature, substantial quantity of ZnO nanowires was formed. ZnO is a polar crystal, where zinc and oxygen atoms are arranged alternatively along the c-axis and the top surface is Zn-terminated  while the bottom surface is oxygen-terminated [18–21]. The top surfaces are Zn-terminated (0001) which are catalytically active, while the bottom surfaces are oxygen-terminated (000ī) which are chemically inert. Consequently, ZnO crystal grows fast along the direction in which the tetrahedron corners point . The growth along the  direction is dominated over other growth facets. This implies that the c-axis is the highest growth direction and the ZnO  has the highest energy of the low-index surface which results in the formation of 1D ZnO nanowires.
Gas Sensing Performance of ZnO Nanowires
In conclusion, single crystal ZnO nanowires were synthesized from high purity Zn metal via an ultra-fast, microwave-assisted process. The major advantage of this technique is its simplicity, low power consumption, fast growth (100 s), and large quantity (in grams) of nanowires. The ZnO nanowires have a wurtzite structure and showed a fast response and high sensitivity to hydrogen gas at 200 °C.
The authors would like to thank KFUPM for its support. Ahsanulhaq Qurashi is thankful to venture business laboratory of Toyama University for post doctoral fellowship.