4. Ethanol sensor performance characteristics

The different electrophysical properties exhibited by semiconductor metal oxides lie in the range from insulators to various band gap materials [20]. The conductivity of semiconducting materials like ZnO normally deviates from stoichiometry [27]. In the measurement of conductivity, vacancies of interstitial ions play an essential part. Sensors of semiconductor metal oxide work as a result of adsorption of gas on the sensing surface that cause in the variation of electrical resistance of ZnO. Due to charge concentration, semiconducting materials are categorized in n-type (like ZnO, TiO2, WOx, MoO3, TiO2, CdO, and SnO2) and p-type (such as NiO, TeO2, and CuO) materials. The species of target gas can also be categorized into two types, oxidizing and reducing gases: oxidizing gas (electron acceptors) like NO2 and O2 and reducing gas (electron donors) such as CO, H2, HCHO, H2S, and C2H5OH. On the exposure of reducing gas on an n-type material surface, it chemisorbed and excess electrons are transferred to the surface of material. As a consequence, the material's resistivity reduced. The adverse effect is investigated in p-type materials. Such type of electrical adaptation was employed for gas sensing.

The important parameters of a sensor are sensitivity, recovery time, operating temperature, response time, and sensing smallest range as described in literature. Everywhere in the literature, sensitivity (S) of a sensor was illustrated in various ways consisting of S = Ra/Rg, S = Rg/Ra, S = ΔR/Rg and S = ΔR/Ra, where Ra is the sensor resistance in ambient air, Rg is the sensor resistance in the target gas, and ΔR = |Ra � Rg| [28, 66]. The investigation shows that sensitivity values are described as introduces by the author. The formula for measurement of sensitivity is also specifying. Response time can be stated as the time needed for a sensor to attain the original baseline 90% of the final response of the signal-like resistance on flow of required gas. The sensor recovery time can be stated as the time required returning 90% of the final signal on removal of target gas.

## 5. Design of gas sensor with 1D nanostructures

In the designing of nanogas sensors, 1D nanoarchitectures are used that contain metal oxide semiconductors in the structure of nanowires, nanotubes, nanorods, nanobelts, nanowhiskers, nanofibers, nanoneedles, nanoribbons, nanopushpins, fiber-mats, lamellar, urchin, and in the form of hierarchical dendrites. In all these structures, nanowires, nanotubes, nanofibers, and nanorods are rod-form nanoarchitectures with a diameter ranging from 1 nm to 200 nm. Nanowires and nanorods have aspect ratios from 2 to 20 and more than 20 [67]. Anyhow, the aspect ratio of nanofibers is greater than that of nanowires. Primarily, nanotubes are hollow nanorods with a determined thickness of walls. The interpretation of nanoarchitectures like nanowhiskers [25], fiber-mats [22], nanoribbons [24],

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

nanobelts [16], urchin [26], nanoneedles [23], lamellar [28], nanopushpins [27], and hierarchical dendrites [17] can be established in the appropriate literatures. It is essential to indicate that the difference among various nanoarchitectures is not forever understandable and is mostly conditions used alternately from one to another reference.

To design sensor, nanoarchitectures are arranged in various forms. Normally, nanostructures are arranged in electrode attachment technique. Generally, adjustment of nanostructures is divided into three forms such as: (a) single arrangement of nanostructures, (b) aligned, and (c) random adjustment. It has been investigated that in the detection of different gases like hydrogen single adjustment of nanofiber was used by researchers [68–70]. Due to various aspect ratios, the nanostructures may be in the form of nanowires or nanorods [5].

An in-situ lift-out method has been investigated by Lupan et al. to detect hydrogen gas for preparing single ZnO nanorod sensor. An electropolished tungsten wire is joined to a single ZnO nanorod, which is connected to external electrodes. In the way Lupan et al. [71, 72] and Hwang I-S [64] has also designed single tripod and tetrapod sensors with the help of in-situ lift-out process by FIB. For nano-/ microsensors depending upon nanostructures from semiconductor metal oxides, their process gained a 90% progress rate.

Liao et al. fabricate a gas sensor for ethanol detection in which zinc oxide nanorod arrays were used inside an indium thin film and silicon substrate [71, 72]. Ohmic contact is provided by an indium film, and for electrode, copper sheet was used. Arbitrary separated nanostructured sensors have alterations such as (a) distribution of nanostructures randomly in the film shape, (b) arbitrary separation of nanostructures' drop on the circumference of a tube, and (c) random distribution of nanostructures pressed in tablet shape. Flat interdigitated substrate was used by Wan et al. where arbitrary distribution of zinc oxide nanowires were dissolved by ultrasonication in ethanol and then spin coated on silicon interdigitated substrate. Occasionally, fabrication and attachment of nanowires with substrate are integrated with the device construction [73–75].

The tube-shaped sensors are modification of film-shaped arbitrary distributed nanostructured sensors; hence, the smooth surface is the form to a tube. Such kind of sensor has a ceramic tube-type substrate. Normally, Al2O3 is used as tube material and the surface is coated with one-dimensional gas sensing materials. Different types of one-dimensional gas sensing materials having various morphologies may be used on the surface of the tube. Hao et al. [26] designed a tube-shape sensor for H2S sensing.

To design tablet-shaped sensors, randomly separated nanostructures can be used. Ethanol sensing was done by such detectors as reported by Zhou et al. [47]. At 6 MPa pressure, ZnO nanorods were grown in pellets form with thickness of 5.3 cm2 areas and dimension of 3 mm. Electrodes were made from silver paste at the back and front surface of the ZnO pellets.

#### 5.1 Gas detection from nanostructural materials

Nowadays, due to fine crystallinity, high aspect ratio and charge detection capability of one-dimensional nanostructure materials have become intensified for gas detection applications.

Various routes have been adopted for the synthesis of 1D nanostructures for gas detectors. The cost, yield, quality of the materials, and complexity differ for various processes. The metal oxides TiO2, ZnO, WOx, SnO2, CdO, In2O3, CuO, Fe2O3, AgVO3, TeO2, and MoO3 have been studied for various required gases with

diffraction (SAED) analysis were obtained by JEOL JEM-2010 microscope that was operating at an accelerating voltage of 200 kV. Photoluminescence (PL) spectrum was determined by Hitachi F-7000 FL Spectrophotometer with 325 nm excitation range at room temperature from Xe lamp. X-ray photoelectron spectrometry (XPS) was done by using Al Kα (hν = 1486.6 eV) with X-ray beams as the excitation source. Binding energies were calibrated relative to the C1s peak at 284.6 eV. The specific surface areas were measured via the Brunauer-Emmett-Teller (BET) method using an N2 adsorption at 77 K after treating the samples at 100°C and 10–<sup>4</sup> Pa for 2 h using a

The different electrophysical properties exhibited by semiconductor metal oxides lie in the range from insulators to various band gap materials [20]. The conductivity of semiconducting materials like ZnO normally deviates from stoichiometry [27]. In the measurement of conductivity, vacancies of interstitial ions play an essential part. Sensors of semiconductor metal oxide work as a result of adsorption of gas on the sensing surface that cause in the variation of electrical resistance of ZnO. Due to charge concentration, semiconducting materials are categorized in n-type (like ZnO, TiO2, WOx, MoO3, TiO2, CdO, and SnO2) and p-type (such as NiO, TeO2, and CuO) materials. The species of target gas can also be categorized into two types, oxidizing and reducing gases: oxidizing gas (electron acceptors) like NO2 and O2 and reducing gas (electron donors) such as CO, H2, HCHO, H2S, and C2H5OH. On the exposure of reducing gas on an n-type material surface, it chemisorbed and excess electrons are transferred to the surface of material. As a consequence, the material's resistivity reduced. The adverse effect is investigated in p-type materials. Such type of electrical adaptation was employed for gas sensing. The important parameters of a sensor are sensitivity, recovery time, operating temperature, response time, and sensing smallest range as described in literature. Everywhere in the literature, sensitivity (S) of a sensor was illustrated in various ways consisting of S = Ra/Rg, S = Rg/Ra, S = ΔR/Rg and S = ΔR/Ra, where Ra is the sensor resistance in ambient air, Rg is the sensor resistance in the target gas, and ΔR = |Ra � Rg| [28, 66]. The investigation shows that sensitivity values are described as introduces by the author. The formula for measurement of sensitivity is also specifying. Response time can be stated as the time needed for a sensor to attain the original baseline 90% of the final response of the signal-like resistance on flow of required gas. The sensor recovery time can be stated as the time required

Tristar-3000 apparatus [38].

Gas Sensors

4. Ethanol sensor performance characteristics

returning 90% of the final signal on removal of target gas.

In the designing of nanogas sensors, 1D nanoarchitectures are used that contain metal oxide semiconductors in the structure of nanowires, nanotubes, nanorods, nanobelts, nanowhiskers, nanofibers, nanoneedles, nanoribbons, nanopushpins, fiber-mats, lamellar, urchin, and in the form of hierarchical dendrites. In all these

structures, nanowires, nanotubes, nanofibers, and nanorods are rod-form

nanoarchitectures with a diameter ranging from 1 nm to 200 nm. Nanowires and nanorods have aspect ratios from 2 to 20 and more than 20 [67]. Anyhow, the aspect ratio of nanofibers is greater than that of nanowires. Primarily, nanotubes are hollow nanorods with a determined thickness of walls. The interpretation of nanoarchitectures like nanowhiskers [25], fiber-mats [22], nanoribbons [24],

5. Design of gas sensor with 1D nanostructures

70

different sensitivity. Here, we discuss ZnO 1D nanostructures in terms of their fabrication, characterization, and sensitivity for sensing ethanol.

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

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

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

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 com-

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 ZnO-

based 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].

into and out of the sensing materials than did the dense structures [65].

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].

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

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 temper-

The SAW sensor for ethanol detection constructed from ZnO nanorods exhibits

of ZS-10 consisting of 10 wt% Sm2O3 is investigated.

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

pure ZnO sensor [56].

mercialization [57].

ature of 370°C [58].

73
