Abstract

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, main 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 CH4 sensing of ZnO nanostructures. ZnO nanowires showed a remarkable sensing response toward acetone and CH4 gas.

Keywords: metal oxide, semiconductor material, Mg doping, nanobelts, structural and morphological properties, band gap, gas sensing

### 1. Introduction

From the last decade, nanotechnology has established a bridge among all the fields of science and technology. Low-dimensional materials and structures have exceptional properties that make them able to play a critical role in the rapid progress of field science. With these excellent properties, 1-D metal oxide semiconductors (MOS) have become the backbone of research in all fields of natural sciences.

Nanotechnology deals with structures and materials of very small dimensions. Nanotechnology is the foundation and exploitation of nanomaterial with structural features in between those of atoms and their bulk material. The properties of the materials at nanoscale are extensively different from those of bulk materials. The high surface reactivity with the surrounding surface improves significantly. When the size of materials is in the nanoscale, the surface-area-to-volume-ratio (L/D) becomes large that makes the nanomaterial ideally an appropriate candidate for many types of sensing applications. That is why nanomaterial has opened up possibilities for new pioneering functional devices and technologies. Nanostructures have at least one dimension less than 100 nm. Crystal structures are much stable at nanoscale [1].

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.

2.1 Structural properties of ZnO

DOI: http://dx.doi.org/10.5772/intechopen.86815

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

2.2 Crystal and surface structure of ZnO

presented below.

Figure 1.

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ZnO is a key technological and prominent material. One of the important properties of ZnO is that it has a wide band gap that makes it suitable for optoelectronic applications of short wavelength. ZnO has high excitonic binding energy (60 meV) at room temperature by ensuring efficient excitonic emission. It has been noted that disordered nanoparticles and thin films at room temperature have ultraviolet (UV) luminescence. In addition, due to the unavailability of centrosymmetry in wurtzite structures that combines with large electromechanical coupling which result in strong piezoelectric and pyroelectric properties and make ZnO a prominent material in the use of mechanical actuators and piezoelectric sensors. As a versatile functional material, ZnO has a different group of growth morphologies, such as nanocombs, nanowires, nanobelts, nanosprings, etc. These ZnO nanostructures are easily obtained, even on cheap substrates such as glass. As work done in this thesis mainly deals with ZnO semiconductor, structural properties of this material are

At normal temperature and pressure, ZnO crystallizes in wurtzite (B4 type) structure, as shown in Figure 1. It is a hexagonal lattice, belonging to the space group P63mc with lattice parameters a = 0.3296 nm and c = 0.52065 nm. The tetrahedral coordination in ZnO is responsible for noncentral symmetric structure and consequently results in piezoelectricity and pyroelectricity. Another important characteristic of ZnO is polar surfaces. The most common polar surface is the basal plane. The oppositely charged ions produced positively charged Zn+ (0001) and negatively charged O (0001) surfaces, which result in a normal dipole moment and spontaneous polarization along the c-axis as well as variance in surface energy. The two most commonly observed facets for ZnO are (2110) and (0110) which are nonpolar surfaces and have lower energy than the (0001) facets [14, 15]. ZnO has varied properties, covering all of its physical, chemical, or material properties.

(a) Crystal structure of hexagonal wurtzite ZnO, ZnO unit cell, including the tetrahedral coordination between Zn and its neighboring O. (b) ZnO has a noncentro-symmetric crystal structure that is made up of alternate

layers of positive and negative ions, leading to spontaneous polarization.

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 [3–11].

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 [12–21].

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 (SnO2 nanowhiskers, In2O3 nanowires, ZnO nanorods, WO3 nanowires, TiO2 nanowires etc.) have been fabricated into film-type nanosensors by means of thermal evaporation or vapor transport method. The most widely studied substances are SnO2 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].

### 2. 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.
