6. Performance of ethanol sensors

The ZnO NRs were then used to construct a gas sensor for ethanol sensing at various working temperature from 200 to 340°C. The as-prepared ZnO nanorod gas sensor exhibited a high, reversible, and fast response to ethanol, indicating its potential application as a gas sensor to detect ethanol. It is obvious that with the increase of ethanol concentration the response increases [29].

Flexible ZnO nanorod sensors were examined to monitor ethanol gas by varying the working temperature from 300°C to by changing the ethanol concentration in a range from 100 to 10 ppm in synthetic air. The sensitivity of a gas sensor is not influenced by the thickness of the seed layer, which presents that density of the charge carrier of ZnO NR is responsible for gas detection properties. The dimension of NRs plays an essential part in evaluating the sensitivities. A flexible ZnO nanorod sensor can detect 100 ppm of ethanol gas with a sensitivity of 3.11 at 300°C [42].

Due to large aspect ratio, the response of ZnO NRs to ethanol gas is better than that of the ZnO seed layer sensor. There are more free electrons taking part in the sensing processes with adsorbed oxygen species for the NRs grown with Zn salt/ KOH solution. Therefore, a better sensitivity of the NRs grown with Zn salt/KOH solution to ethanol gas is mainly a result of the more free carriers originating from oxygen vacancies in sensor materials. The Schottky or ohmic contact shows that Au has more work function than ZnO. Hence, the sensor exhibiting the Schottky contact has a better response to ethanol gas than possessing an ohmic behavior. In conclusion, the free electrons are an important parameter for increasing the sensor response beside large aspect ratio [41].

The response of ZnO NRs decorated with photochemical Ag NP sensor to 50 ppm ethanol is almost three times as high as that of those made from pure-ZnO NRs. The electronic sensitization of noble metal doping like Ag is comfortable to grow stable oxide (Ag2O) at operating temperature (280°C) in air. The adsorbed O2 on the surface of Ag2O extracts electron from ZnO and produces a depletion layer. After exposing to some target gases, Ag2O reduced to Ag and eventually gives ZnO. Hence, due to increase in charge carrier, the sensitivity of the sensor enhanced. The responses of the sensors have no apparent degradation after being exposed to ethanol of 30 ppm for 100 days [54].

The doped ZnO sensors show increased sensitivity as compared to undoped conditions. It is shown that the sensitivity was highly affected by both Co doping and its concentration. The sensor response has been boosted by a factor of 1.6 and 1.8 for 1.85 at.% Co (Sg = 10.9) in comparison with 0.76 at.% (Sg = 6.7) and by undoped conditions (Sg = 6) in ZnO NRs, respectively, for 50 ppm C2H5OH [44].

The response of gas sensor based on n-type semiconductor ZnO nanorods exposed to various concentrations of the ethanol vapors at 400°C has been investigated. It showed a considerably high response even at low concentration of 5 ppm ethanol. Their response and recovery times were less than 10 and 30 s, respectively. In addition, the sensors were still sensitive to 5 ppm ethanol, even after exposure to 300 ppm ethanol. The reversible cycles of the response curve indicate a stable and repeatable operation of gas sensing. The much higher sensitivity may be due to the large effective surface areas. The results revealed that ZnO NRs show excellent response and stability. The higher response may be due to greater surface area [35].

#### A Review on Preparation of ZnO Nanorods and Their Use in Ethanol Vapors Sensing DOI: http://dx.doi.org/10.5772/intechopen.86704

The resistance of the sensor significantly enhanced in air at the small temperature of 200°C, due to samaria contents. A 10 times further increase in the resistance of ZS-10 consisting of 10 wt% Sm2O3 is investigated.

At the small temperature of 200°C, the sensor resistance in air significantly increases with samaria contents. A 10 times more increase in the resistance of ZS-10 consisting of 10 wt% Sm2O3 is investigated. At greater value of temperatures from 250 to 350°C, the ZS-5 and ZS-10 have the same resistances, yet greater than those of ZS-2 and Z. The main work of samaria is perhaps placed on the ZnO surface, averting direct interaction with ZnO nanoparticles that as a result enhanced the Schottky barrier. The analysis of ZnO by 10 wt% Sm2O3 is perhaps liable for the greater resistance explored for this sample. These investigations are explained with DRS UV-vis. The sensitivity of the Sm2O3-ZnO sensor was observed in the presence of 500 ppm of CO, toluene, and ethanol and 1.0 vol% of methane at a temperature of 200–400°C in air for Sm2O3. A significant increase in the ethanol response was investigated at different temperatures due to 5 wt% doping of Sm2O3-ZnO, although a smaller one toward toluene, methane, and CO. A 60 times greater response was observed to ethanol with 5 wt% Sm2O3-ZnO as compared to that of pure ZnO sensor [56].

A rapid and repeatable detection was observed toward ethanol vapors at normal temperature and 330°C. At an operating temperature between 280 and 310°C, the sensor has excellent response with a detection time to 500 ppm ethanol and in a time from 16 s to 120 s. High sensitivity, fast recovery, and response time have been explored at this temperature limit. The results investigate that the constructed sensor is a potential candidate for industrial applications and promising for commercialization [57].

The research explores the ZnO growth environment that explains the properties related to opto-geometry of the random ZnO nanorods over layer and the spectral response is dominating over the long time grating device. The response of ethanol toward ZnO-overlaid LPGs was confirmed under various fabrication conditions and also their spectral response correlating with available simulation models [59].

The sensor resistance enhanced with the flow of ethanol on the surface of the sensor. The sensor response time (which is defined as the 90% of the full response) is small and changes from 200 to 125 s as the concentrations enhanced from 809 to 4563 ppm, and it was investigated that the recovery time is approximately 10 min [60].

The result presents that high sensing characteristic has been established by ZnObased sensor in the ethanol concentration (1–99.5%). The conduction process depends upon internal resistance of the nanorods for small value of ethanol concentration (a semicircle at high frequency). The charge transfer resistance interior the grain boundaries shows a straight line at small frequency region. I-V characteristics determine the sensitivity of ZnO sensor. ZnO NR arrays have potential to fabricate a chemical sensor with small power consumption and high sensitivity [49].

The recovery time response and response were established in the range of 15 and 5 s for 95% of total recovery and response, respectively. The porous structure of the sensing material is responsible to this phenomenon. The molecules diffused easily into and out of the sensing materials than did the dense structures [65].

The response enhanced from 7 to 9 with increase in temperature from 160 to 300°C. Hence, it enhanced drastically when temperature attains a value of more than 300°C. The value of the response varies from 9 to 24 as the temperature changes from 300 to 340°C and then attains the maximum value of 34 at a temperature of 370°C [58].

The SAW sensor for ethanol detection constructed from ZnO nanorods exhibits a frequency of 24 kHz at 270°C to 2300 ppm ethanol response, which is an improvement of 9 kHz of the sensor without nanorods [65].

different sensitivity. Here, we discuss ZnO 1D nanostructures in terms of their

The ZnO NRs were then used to construct a gas sensor for ethanol sensing at various working temperature from 200 to 340°C. The as-prepared ZnO nanorod gas sensor exhibited a high, reversible, and fast response to ethanol, indicating its potential application as a gas sensor to detect ethanol. It is obvious that with the

Flexible ZnO nanorod sensors were examined to monitor ethanol gas by varying the working temperature from 300°C to by changing the ethanol concentration in a range from 100 to 10 ppm in synthetic air. The sensitivity of a gas sensor is not influenced by the thickness of the seed layer, which presents that density of the charge carrier of ZnO NR is responsible for gas detection properties. The dimension of NRs plays an essential part in evaluating the sensitivities. A flexible ZnO nanorod sensor can detect 100 ppm of ethanol gas with a sensitivity of 3.11 at 300°C [42]. Due to large aspect ratio, the response of ZnO NRs to ethanol gas is better than that of the ZnO seed layer sensor. There are more free electrons taking part in the sensing processes with adsorbed oxygen species for the NRs grown with Zn salt/ KOH solution. Therefore, a better sensitivity of the NRs grown with Zn salt/KOH solution to ethanol gas is mainly a result of the more free carriers originating from oxygen vacancies in sensor materials. The Schottky or ohmic contact shows that Au has more work function than ZnO. Hence, the sensor exhibiting the Schottky contact has a better response to ethanol gas than possessing an ohmic behavior. In conclusion, the free electrons are an important parameter for increasing the sensor

The response of ZnO NRs decorated with photochemical Ag NP sensor to 50 ppm ethanol is almost three times as high as that of those made from pure-ZnO NRs. The electronic sensitization of noble metal doping like Ag is comfortable to grow stable oxide (Ag2O) at operating temperature (280°C) in air. The adsorbed O2 on the surface of Ag2O extracts electron from ZnO and produces a depletion layer. After exposing to some target gases, Ag2O reduced to Ag and eventually gives ZnO. Hence, due to increase in charge carrier, the sensitivity of the sensor enhanced. The responses of the sensors have no apparent degradation after being exposed to

The doped ZnO sensors show increased sensitivity as compared to undoped conditions. It is shown that the sensitivity was highly affected by both Co doping and its concentration. The sensor response has been boosted by a factor of 1.6 and 1.8 for 1.85 at.% Co (Sg = 10.9) in comparison with 0.76 at.% (Sg = 6.7) and by undoped conditions (Sg = 6) in ZnO NRs, respectively, for 50 ppm C2H5OH [44]. The response of gas sensor based on n-type semiconductor ZnO nanorods exposed to various concentrations of the ethanol vapors at 400°C has been investigated. It showed a considerably high response even at low concentration of 5 ppm ethanol. Their response and recovery times were less than 10 and 30 s, respectively. In addition, the sensors were still sensitive to 5 ppm ethanol, even after exposure to 300 ppm ethanol. The reversible cycles of the response curve indicate a stable and repeatable operation of gas sensing. The much higher sensitivity may be due to the large effective surface areas. The results revealed that ZnO NRs show excellent response and stability. The higher response may be due to greater surface area [35].

fabrication, characterization, and sensitivity for sensing ethanol.

increase of ethanol concentration the response increases [29].

6. Performance of ethanol sensors

Gas Sensors

response beside large aspect ratio [41].

ethanol of 30 ppm for 100 days [54].

72

It was investigated that a large surface energy and greater amount of O2 vacancies' concentration exist in the NA and NHS surface that increased the ethanol detection characteristics at small temperature [53].

The conductivity of Ti-ZnO sensor was greater than that of the pure ZnO NRs by a factor of 80.7 that is 1.86%. The ethanol gas sensing response of the ZnO:Ti NR sensor has a linear relationship with temperature. The responses were found at 27.5, 66.7, 117.1, 183.5, 276.5, and 389.5% with 10, 50, 100, 250, 500, and 1000 ppm ethanol concentration, respectively. It was investigated that the responses of ZnO:Ti NR sensor are greater than those of the ZnO NRs at 1000 ppm ethanol concentration by about 5.1 [61].

It was demonstrated that they exhibited good performance for detecting ethanol vapor even at 380 and 250°C [48].

It is confirmed from the experimental results that there is no noticeable degradation of the sensor response after the flow of 50 ppm ethanol for consecutive 60 days at the operating temperature [63].

The response to 10 ppm NO2 became negligibly small (2.3) compared to that at 200 and 300°C (66.3 and 12.4). This shows that with enhancing temperature at 400°C the capability of C2H5OH sensing increases. The selective detection was investigated with ZnO NWs by 200 ppm C2H5OH as compared to that with ZnO-SnO2 core shell NWs only at a limit of 1/7 [64].

The research results show that from a temperature of 300°C the gas response increases abruptly till it attains the maximum value of 400°C. Then, the response decreases rapidly to a temperature limit of 400–430°C. The smaller value of working temperatures (300–400°C), 400°C, was investigated as the OOT for the ZnO NR sensor with 200 ppm ethanol response was observed to have a value of 193.7 [38].

The addition of detection layers in ZnO shows greater value of sensitivity with greater stability/reproducibility to ethanol, and comparatively small response and recovery time relative to the pure layer of MoO3. Ethanol sensing increased with the impurity (Gethanol/Gair). The 25% ZnO in the presence of MoO3 layer as impurity for 500 ppm ethanol attains a level of 171 under nonhumid air. Its response under humid air is 117 (75% r.H. at 21°C). This response is 6 times greater than that for the pure MoO3 layer [62].

The sample has approximate response and recovery time of 3 and 4 min, respectively. As regard the detection characteristics of other ZnO nanoarchitectures working at greater temperatures, the ZnO NR sensors have high response and recovery time when operated at room temperature [47].

Author details

Musarrat Jabeen<sup>1</sup>

Saidu Sharif Swat, Pakistan

musarrat.physics@gmail.com

provided the original work is properly cited.

Pakistan

75

\*, R. Vasant Kumar<sup>2</sup> and Nisar Ali<sup>3</sup>

A Review on Preparation of ZnO Nanorods and Their Use in Ethanol Vapors Sensing

DOI: http://dx.doi.org/10.5772/intechopen.86704

3 Department of Physics Government Post Graduate, Jahanzeb College,

\*Address all correspondence to: musarrat97@yahoo.com;

1 Department of Physics, Government Degree College for Women, Haveli Lakha,

2 Department of Material Science and Metallurgy, University of Cambridge, UK

© 2020 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,

A Review on Preparation of ZnO Nanorods and Their Use in Ethanol Vapors Sensing DOI: http://dx.doi.org/10.5772/intechopen.86704
