4.8 CH4 gas sensing response

The first step was the preparation of CH4 gas sensor. In the fabrication of CH4 gas sensor, a thick layer (200 nm) of Au was coated by ion sputtering technique on Si (100) wafers. A small amount of ZnO nanostructures was put on a pair of interdigitated electrodes on Si substrates having a gap of 55 μm. A small drop of methanol was dropped on the nanomaterials so that a thick paste was formed. The annealing of sensors was carried out in an open furnace tube for 2 h at 400°C before performing the gas sensing experiments, for the purpose of attachment of oxygen on the surface of sensors. The sensing experiment was performed at 200°C with 5-min cycles of dry air and 400 ppm CH4 gas concentration. The sensing response (S = Ra/ Rg) of the device was measured by resistance change upon exposure to air (Ra) and CH4 gas (Rg). Figure 10(a) and (b) shows the sensitivity response of CH4 (methane gas) at 200°C for undoped ZnO nanowires and for Mg-doped ZnO nanobelts, respectively. Research papers showed that the sensitivity of the resistive sensors is highly affected by the Mg doping. The sensors were tested in a temperature range of 50–200°C for 400 ppm of CH4 gas. Sensors show some response magnitude from 100°C temperature. Undoped ZnO nanowire sensors get its optimal point at 200°C

a huge sensing response to acetone (14), and those of the other solvents are no

The Higher Education Commission (HEC) of Pakistan is acknowledged for financial support through project No. 9294/NRPU/R&D/HEC/2017. Thanks to Prof. Dr. Syed Zafar Ilyas and Dr. Waqar. A. Syed. The authors would also be thankful to

COMSATS University Islamabad for necessary funds for the project.

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

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

There is no conflict of interest in this chapter.

\*, Majeed Gul<sup>2</sup>

\*Address all correspondence to: nabbasqureshi@yahoo.com

1 Thin Films Technology Laboratory, Department of Physics, COMSATS

2 Centre of Excellence in Science and Applied Technologies, Islamabad, Pakistan

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Murrawat Abbas<sup>3</sup> and Muhammad Amin2

greater than 8.6.

Acknowledgements

Conflict of interest

Author details

119

Nazar Abbas Shah<sup>1</sup>

University, Islamabad, Pakistan

3 NUST University, Islamabad, Pakistan

provided the original work is properly cited.

Figure 10.

(a) CH4 gas sensing response of undoped ZnO nanowires (S1). (b) CH4 gas sensing response of Mg-doped ZnO nanobelts.

with response magnitude of 1.84. Doped ZnO nanobelts also get its optimal operating point at 200°C. Its response magnitude was obtained at 2.06. The best sensing signal response of CH4 was found at 200°C. The sensing response at 50, 100, and 150°C (not shown) was comparatively negligible. The sensing response of undoped ZnO nanowires and Mg-doped ZnO nanobelts was found to be 1.84 and 2.06 at 200° C for the same concentration, respectively. The enhanced sensitivity response was observed for the ZnO nanostructures as shown in Figure 10(b). Large amount of oxygen molecules and atoms are adsorbed on Mg-doped ZnO nanobelts due to large surface area (i.e., large defects are created) due to which interaction chance of CH4 gas increases as compared to undoped ZnO nanostructures [60, 61]. On exposing the surface of the ZnO nanostructures to air, oxygen is adsorbed at the ZnO nanostructures surface by capturing an electron from conduction band of surface sites of undoped and Mg-doped ZnO nanostructures [62, 63]. Reactive O2 (oxygen molecules) are chemisorbed or trapped by these ZnO nanostructures from the air, forming active oxygen species O2 and O; as a result transformation of electrons takes place, due to which a wide space charge is formed that results in a decrease in carrier concentration due to which the resistance of the material is increased.

### 5. Conclusions

Growth of 1-D ZnO nanostructures was presented in the present chapter. Vaporliquid-solid mechanism has been employed for the synthesis of ZnO nanostructures. It was found that the morphologies tuned with change in temperature which leads to the formation of nanowires at 850°C, nanorods at 900°C, nanobelts at 950°C, and nanobelts with needle-like ends at 1030°C. The dimensions of the morphologies have been measured by SEM. The length of the structures from 2.93 to 319.48 μm, thickness of the structures from 0.05 to 1.88 μm, and diameter of the structures from 0.95 to 12.66 μm have been obtained successfully. XRD peaks show that the crystallinity and intensity increase with increase in temperature. Doping of magnesium acetate (0.05 g) in ZnO through vapor transport method was successfully achieved. The sensing response of doped ZnO nanostructures for UV light at room temperature and CH4 gas at 200°C has increased. ZnO nanowires show great selectivity response toward different volatile organic compounds (ethanol, methanol, and acetone). At the same concentration and temperature, the ZnO nanowires show Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors DOI: http://dx.doi.org/10.5772/intechopen.86815

a huge sensing response to acetone (14), and those of the other solvents are no greater than 8.6.
