Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors [chapter]

Nazar Abbas Shah, Majeed Gul, Murrawat Abbas, Muhammad Amin
2020 Gas Sensors  
Zinc oxide (ZnO) is a unique and important metal oxide semiconductor for its valuable and huge applications with wide band gap (3:37 eV) and most promising candidate for gas sensor due to its high surface-to-volume ratio, good biocompatibility, stability, and high electron mobility. Due these properties, metal oxide shows good crystallinity, higher carrier mobility, and good chemical and thermal stability at moderately high temperatures. In this chapter nanostructures have been investigated,
more » ... n focus being their synthesis and sensing mechanism of different toxic chemicals, synthesized by thermal evaporation through vapor transport method using vapor-liquid-solid (VLS) mechanism. The doped ZnO nanobelts showed significant enhanced sensing properties at room temperature, indicating that doping is very much effective in improving the methane CH 4 sensing of ZnO nanostructures. ZnO nanowires showed a remarkable sensing response toward acetone and CH 4 gas. Reduction of an object size results in large surface to volume ratio hence the surface turn out to more vital and that large surface to volume ratio greatly affected the chemical, electrical and optical properties of nanomaterials. Quantum effects owing to size confinement in nanostructures occurs, when the typical size of the object is equivalent to the crucial length (range 1-10 nm) of the equivalent physical properties' screening length, then the mean free path of electrons; 0-D quantum dots, 1-D quantum dots, and 2-D quantum well are the characteristic structure forms. Low power consumptions, best crystallinity, and high integration density 1-D with high aspect ratio are shown by the 1-D nanostructures. The nanostructure materials show high sensitivity to surface chemical reactions, with increased surface-to-volume ratio and a Debye length matching with small size. Tunable band gap is enabled by size confinement [2] . In the recent past, various synthesis methods, such as vapor phase method, electrochemical method, liquid phase methods, and solution-gel methods, were used. Out of these growth techniques, vapor transport method, using vapor-liquid-solid (VLS) growth mechanism or VS growth, is one of the finest growth techniques used for the growth of metal oxide semiconductor nanostructures. It is a cost-effective easy method used to create many single-crystalline 1-D nanostructures [3] [4] [5] [6] [7] [8] [9] [10] [11] . Smart and functional materials are based on metal oxides [10]. Synthesis and fabrication of devices based on metal oxide semiconductor have become more important recently, because the tuning of physical properties of these metal oxides is so easy. Among these MOS, ZnO is a material that has strong piezoelectric and optical properties on the bases of its wide band gap, stability at high temperature, large surface-to-volume ratio, and high excitonic binding energy. They are used in solar cells, photocatalysis, and antibacterial active material. Therefore research work has been carried out on ZnO nanostructures. Metal oxide materials possess electrical, chemical, and physical properties that are highly sensitive to the changes in a chemical environment, through a variety of detection principles based on ionic, conducting, photoconducting, piezoelectronic, pyroelectronic, and luminescence properties [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] . Doping is another technique utilized to improve ultraviolet (UV) sensing properties of metal oxides, where the dopant atoms are believed to act as activators for surface reactions. In MOS, the electrical, optical, and chemical properties can be changed by adding the doping materials or by creating oxygen defects which results in large concentration of carriers, mobility, and electrical resistivity. So doping offers another avenue for expanding their sensing capability [12] . Up to now, various metal oxides' 1-D nanostructures (SnO 2 nanowhiskers, In 2 O 3 nanowires, ZnO nanorods, WO 3 nanowires, TiO 2 nanowires etc.) have been fabricated into film-type nanosensors by means of thermal evaporation or vapor transport method. The most widely studied substances are SnO 2 and ZnO nanowires [13] . In this research work, 1-D n-ZnO nanostructures (nanowires, nanorods, nanobelts with needle-like ends, and typical nanobelts) were grown by using vapor transport method using VLS mechanism on n-type Au-coated silicon substrate Si (100). The electrical and optical properties of ZnO nanostructures were investigated using different characterization techniques [14-37]. Important properties of metal oxide semiconductors As work done in this chapter mainly deals with ZnO semiconductor, structural properties of ZnO material are presented below.
doi:10.5772/intechopen.86815 fatcat:52ja2kzgczg3jfsual2xsry56y