3. Characterization

The characterization was done by using various spectroscopic techniques such as X-ray diffraction, scanning electron microscopy, and transmission electron microscopy for structural and morphological analysis.

#### 3.1 X-ray diffraction

X-ray diffraction was done by a diffractometer of Rigaku D/max-2500 with Cu Kα radiation (λ = 0.15418 nm) and 2θ range from 20 to 80°, which confirm the hexagonal wurtzite structure of ZnO (JCPDS card no. 36-1451), the peaks are in agreement in Bragg reflections and no impurity peaks were observed [29].

#### 3.2 Scanning electron microscopy

Surface morphology was done by Shimadzu SS-550 with a voltage of 15 kV. The ZnO rod-like nanostructure affirms from SEM images. High magnification revealed that ZnO nanorods have a length from 1.7 to 2.1 μm and diameter in the range of 90–200 nm.

#### 3.3 Transmission electron microscopy

Transmission electron microscopy (Philips FEI Tecnai 20ST) at an accelerating voltage of 200 kV investigates the detailed structure of ZnO NRs [29].

The orientation of ZnO NRs was examined by X-ray diffractometer (Rigaku D/MAX B) utilizing Cu Kα radiation for post- and preannealing. The operating voltage and current were 40 kV and 40 mA, respectively. The measurement was conducted from 20 to 60<sup>o</sup> in the 2θ range with scanning rate 5o /min. Field emission scanning ELECTRON microscope (FESEM, JEOL JSM-7000F) was utilized to observe the morphology of grown ZnO nanorods. For sensitivity measurement, an ambient environment was controlled inside a chamber by flowing synthetic air.

The resistance was recorded by a Keithley 2400 SourceMeter (Keithley Instruments, Inc.) [42].

The top view of the ZnO NRs and cross-section morphologies were investigated by high resolution scanning electron microscopy (HR-SEM, S4200, HITACHI) and field emission (FE-SEM, XL-40FEG, PHILIPS). The crystal pattern of the NRs was revealed by X-ray powder diffraction (Rigaku MultiFlex) with a scanning step of 0.01° and scan speed of 4°/min. Fluorescence Spectrometer (MFS230) were used to determine the photoluminescent spectra of ZnO NRs at 325 nm excitation. The gas detection properties were investigated by measuring the resistance variation of the sensor in ambient air condition and ethanol mixed environment at 300°C [41].

The morphology of ZnO NR surface was evaluated by scanning electron micros-

The crystalline size and morphology of ZnO NWs were evaluated with fieldemission scanning electron microscopy (FESEM, JEOL 6340F, operated at 5 kV). NW length and diameter were recorded with high-resolution SEM images. The phase analysis was explored with X-ray diffraction (XRD, Philips PW1730 diffrac-

Field-emission scanning electron microscopy (FESEM, JEOL model JSM-6700 F) was employed to evaluate the cross-sectional images and morphology of ZnO NRs. The NRs are distributed over the selected area uniformly and have hexagonal wurtzite structure with average length of 1.5 μm and diameter is about 55 nm. The crystalline pattern was recorded by X-ray diffraction using Cu Kα radiation. The electrical impedance spectroscopy (EIS) was done by Hioki LCR meter (model: 3532-50). EIS has AC signal amplitude of 50 mV with the frequency range from 42 Hz to 1 MHz. A two-electrode method was applied to determine the response of the ZnO NR sensor (working electrode) and counterelectrode as ITO was employed

The characterization such as X-ray powder diffraction (XRD, D/MAX2500, and

The effective limit of the sensor was found to be typically high (190–1530 ppm ethanol in air), for both kind of modified and unmodified NRs, with a saturation

X-ray diffraction (XRD, D8 Advance Bruker) analysis was conducted to calculate the crystal pattern with Cu Kα1 radiation (λ = 0.15406 nm) that operated at 40 kV. Field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F) was performed to explore the surface morphology of the samples operating at a voltage of 5 kV. Transmission electron microscopy (TEM) pattern and selected area electron

Cu Kα radiation), scanning electron microscopy (SEM, JEOL JSM6700F), and transmission electron microscopy (TEM, JEOL 2010) determines the crystal pattern and surface analysis. The growth technique of the sensors based on the products has been described elsewhere [11, 12]. Briefly, the products were dispersed in ethanol, and a drop was spun on a ceramic tube between Pt electrodes to form a thin film (about tens of nanometers). A resistance heater in the ceramic tube was used to control the temperature. The sensitivity is defined as S = Ra/Rg, where Ra is the sensor resistance in air and Rg is the resistance in the target-air mixed gas [34]. Crystal pattern of the annealed ZnO NRs was characterized by X-ray diffractometer (XRD, DX-2700) using Cu Kα radiation (λ = 0.15418 nm) with 2θ range from 20 to 80°. The morphology, dimension, and surface details of the NRs were explored by scanning electron microscopy (FEI Quanta 200) that operated at 20 kV. The photoluminescence spectrum (PL, PE LS55 spectrophotometer) was recorded with 325 nm excitation from Xe lamp at room temperature. The sensing was done using a sensitivity instrument WS-30A (Wei Sheng Electronics Co. Ltd., China)

copy (SEM, FEI Nova Nano). X-ray diffraction (XRD) recorded the crystalline pattern and chemical composition of the seed layer as well as NRs deposited on the substrate. XRD was done by Bruker D8 Discover microdiffractometer equipped with a GADDS (general area detector diffraction system). The XRD was filtered with a graphite monochromator in the parallel mode (175 mm collimator with 0.5 mm pinholes) that operated with a potential of 40 kV and a current of 40 mA. Data were recorded using Cu Kα radiation (λ = 1.54178 Å) at room temperature. The electrical characterization like conductivity of the sensor was measured with a multimeter (Keithley 2001) with various concentrations of ethanol in synthetic air. The information in real time was collected by data acquisition card attached to the computer with LabVIEW software. The gas mixture was supplied at the rate of 200

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

sccm and sensing done at a temperature range of 25–330°C [57].

tometer) with Cu Kα radiation [60].

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

with a thin coating of Au [49].

with gas detection chamber of 30 L [58].

69

tendency beyond 1530 ppm ethanol (in air) [36].

A transmission electron microscopy (TEM: JEM-200CX, 160 kV, and HR-TEM: JEOL-2010F, 200 kV) was used to visualize the morphology of the grown Ag-ZnO NRs. 0.1 g of Ag-ZnO nanorod was ultrasonically suspended in ethanol 10 mL solution for TEM investigations. Then one drop of this solution is put on a copper grid with thin coating of carbon that evaporated at room temperature in air. An EDX analyzer (EDX: INCA OXFORD) was connected to the JEOL-2010F HR-TEM for in situ investigation of the configuration of the fabricated Ag-ZnO NRs. The elemental composition of Ag-ZnO NRs was further investigated by X-ray photoelectron spectrometer (XPS, PHI 5000C ESCA System, Perkin Elmer) with Mg Kα (λ = 1253.6 eV). The data investigation was done by RBD Auger Scan 3.21 software. Differential scanning calorimetry (DSC) analysis was carried out on a STA409PC thermal analyzer (Netzsch, Germany) with 10°C/min heating rate in air. The phase analysis of NRs was identified with powder X-ray diffraction analysis using a D/max 2550 V diffractometer with Cu Kα radiation (λ = 1.54056 Å) (Rigaku, Tokyo, Japan). One typical sensor containing Ag-ZnO NRs was further visualized by scanning electron microscopy (SEM: JSM-6700F, 10 kV) after working for 100 days [54].

The surface morphology visualization of the final product was investigated with a field-emission scanning electron microscopy (FESEM, JEOL JSM-6500F). The electrodeposited NRs have cross-sectional SEM image with 100–300 nm diameter and 1–2 μm in length. TEM low magnification image confirmed that ZnO NRs are well aligned in nature. Selected area electron diffraction (SAED) indicates that NRs are single crystalline ZnO with wurtzite structure. Energy dispersive analysis confirms the presence of Zn, O, Co, and Cu elements [44].

The crystal pattern of the ZnO NRs was revealed with X-ray diffraction (XRD, D8 Advance Bruker) with incident monochromatized radiation Cu Kα (λ = 1.5418 Å). XRD pattern was measured with a scanning step of 0.02°/s from 20° to 80° (2θ). Scanning electron microscopy (SEM, JSM-6301F) recorded the sizes of the samples and surface morphologies. SEM was operated at 20 KeV and transmission electron microscopy (TEM) investigation was done by H-800 transmission electron microscope that operated at 200 kV. The photoluminescence (PL) analysis was recorded at 325 nm (20 mW) by excitation with a laser emitter He-Cd at room temperature. The gas detection was determined by a China sensitivity instrument HW-30A. The gas response was evaluated by Ra/Rg, where Ra is the resistance in ambient air and Rg is the resistance in tested gas atmosphere [35].

Scanning electron microscopy (SEM, Holland Philips XL30) evaluates the surface morphology and crystalline size of Sm2O3-loaded ZnO samples. The EDX analysis evaluates the elemental composition of the synthesized samples. The crystalline pattern is recorded by the X-ray powder diffraction (XRD, Holland Philips) with Xpert diffractometer (CoKα = 1.7889 Å). The CHEMBET 3000 instrument was employed to record the specific surface areas (SSA) of the fabricated samples defaced at 330°C for 1 h utilizing Brunauer-Emmett-Teller (BET) technique. Diffuse reflectance UV-vis analysis for band gap evaluation was evaluated by an Avantes Avaspec-2048-TEC (using BaSO4 as a reference) [56].

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

The morphology of ZnO NR surface was evaluated by scanning electron microscopy (SEM, FEI Nova Nano). X-ray diffraction (XRD) recorded the crystalline pattern and chemical composition of the seed layer as well as NRs deposited on the substrate. XRD was done by Bruker D8 Discover microdiffractometer equipped with a GADDS (general area detector diffraction system). The XRD was filtered with a graphite monochromator in the parallel mode (175 mm collimator with 0.5 mm pinholes) that operated with a potential of 40 kV and a current of 40 mA. Data were recorded using Cu Kα radiation (λ = 1.54178 Å) at room temperature. The electrical characterization like conductivity of the sensor was measured with a multimeter (Keithley 2001) with various concentrations of ethanol in synthetic air. The information in real time was collected by data acquisition card attached to the computer with LabVIEW software. The gas mixture was supplied at the rate of 200 sccm and sensing done at a temperature range of 25–330°C [57].

The crystalline size and morphology of ZnO NWs were evaluated with fieldemission scanning electron microscopy (FESEM, JEOL 6340F, operated at 5 kV). NW length and diameter were recorded with high-resolution SEM images. The phase analysis was explored with X-ray diffraction (XRD, Philips PW1730 diffractometer) with Cu Kα radiation [60].

Field-emission scanning electron microscopy (FESEM, JEOL model JSM-6700 F) was employed to evaluate the cross-sectional images and morphology of ZnO NRs. The NRs are distributed over the selected area uniformly and have hexagonal wurtzite structure with average length of 1.5 μm and diameter is about 55 nm. The crystalline pattern was recorded by X-ray diffraction using Cu Kα radiation. The electrical impedance spectroscopy (EIS) was done by Hioki LCR meter (model: 3532-50). EIS has AC signal amplitude of 50 mV with the frequency range from 42 Hz to 1 MHz. A two-electrode method was applied to determine the response of the ZnO NR sensor (working electrode) and counterelectrode as ITO was employed with a thin coating of Au [49].

The characterization such as X-ray powder diffraction (XRD, D/MAX2500, and Cu Kα radiation), scanning electron microscopy (SEM, JEOL JSM6700F), and transmission electron microscopy (TEM, JEOL 2010) determines the crystal pattern and surface analysis. The growth technique of the sensors based on the products has been described elsewhere [11, 12]. Briefly, the products were dispersed in ethanol, and a drop was spun on a ceramic tube between Pt electrodes to form a thin film (about tens of nanometers). A resistance heater in the ceramic tube was used to control the temperature. The sensitivity is defined as S = Ra/Rg, where Ra is the sensor resistance in air and Rg is the resistance in the target-air mixed gas [34].

Crystal pattern of the annealed ZnO NRs was characterized by X-ray diffractometer (XRD, DX-2700) using Cu Kα radiation (λ = 0.15418 nm) with 2θ range from 20 to 80°. The morphology, dimension, and surface details of the NRs were explored by scanning electron microscopy (FEI Quanta 200) that operated at 20 kV. The photoluminescence spectrum (PL, PE LS55 spectrophotometer) was recorded with 325 nm excitation from Xe lamp at room temperature. The sensing was done using a sensitivity instrument WS-30A (Wei Sheng Electronics Co. Ltd., China) with gas detection chamber of 30 L [58].

The effective limit of the sensor was found to be typically high (190–1530 ppm ethanol in air), for both kind of modified and unmodified NRs, with a saturation tendency beyond 1530 ppm ethanol (in air) [36].

X-ray diffraction (XRD, D8 Advance Bruker) analysis was conducted to calculate the crystal pattern with Cu Kα1 radiation (λ = 0.15406 nm) that operated at 40 kV. Field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F) was performed to explore the surface morphology of the samples operating at a voltage of 5 kV. Transmission electron microscopy (TEM) pattern and selected area electron

The resistance was recorded by a Keithley 2400 SourceMeter (Keithley

firms the presence of Zn, O, Co, and Cu elements [44].

The crystal pattern of the ZnO NRs was revealed with X-ray diffraction (XRD,

(λ = 1.5418 Å). XRD pattern was measured with a scanning step of 0.02°/s from 20° to 80° (2θ). Scanning electron microscopy (SEM, JSM-6301F) recorded the sizes of the samples and surface morphologies. SEM was operated at 20 KeV and transmission electron microscopy (TEM) investigation was done by H-800 transmission electron microscope that operated at 200 kV. The photoluminescence (PL) analysis was recorded at 325 nm (20 mW) by excitation with a laser emitter He-Cd at room temperature. The gas detection was determined by a China sensitivity instrument HW-30A. The gas response was evaluated by Ra/Rg, where Ra is the resistance in

Scanning electron microscopy (SEM, Holland Philips XL30) evaluates the surface morphology and crystalline size of Sm2O3-loaded ZnO samples. The EDX analysis evaluates the elemental composition of the synthesized samples. The crystalline pattern is recorded by the X-ray powder diffraction (XRD, Holland Philips) with Xpert diffractometer (CoKα = 1.7889 Å). The CHEMBET 3000 instrument was employed to record the specific surface areas (SSA) of the fabricated samples defaced at 330°C for 1 h utilizing Brunauer-Emmett-Teller (BET) technique. Diffuse reflectance UV-vis analysis for band gap evaluation was evaluated by an

D8 Advance Bruker) with incident monochromatized radiation Cu Kα

ambient air and Rg is the resistance in tested gas atmosphere [35].

Avantes Avaspec-2048-TEC (using BaSO4 as a reference) [56].

68

The top view of the ZnO NRs and cross-section morphologies were investigated by high resolution scanning electron microscopy (HR-SEM, S4200, HITACHI) and field emission (FE-SEM, XL-40FEG, PHILIPS). The crystal pattern of the NRs was revealed by X-ray powder diffraction (Rigaku MultiFlex) with a scanning step of 0.01° and scan speed of 4°/min. Fluorescence Spectrometer (MFS230) were used to determine the photoluminescent spectra of ZnO NRs at 325 nm excitation. The gas detection properties were investigated by measuring the resistance variation of the sensor in ambient air condition and ethanol mixed environment at 300°C [41]. A transmission electron microscopy (TEM: JEM-200CX, 160 kV, and HR-TEM: JEOL-2010F, 200 kV) was used to visualize the morphology of the grown Ag-ZnO NRs. 0.1 g of Ag-ZnO nanorod was ultrasonically suspended in ethanol 10 mL solution for TEM investigations. Then one drop of this solution is put on a copper grid with thin coating of carbon that evaporated at room temperature in air. An EDX analyzer (EDX: INCA OXFORD) was connected to the JEOL-2010F HR-TEM for in situ investigation of the configuration of the fabricated Ag-ZnO NRs. The elemental composition of Ag-ZnO NRs was further investigated by X-ray photoelectron spectrometer (XPS, PHI 5000C ESCA System, Perkin Elmer) with Mg Kα (λ = 1253.6 eV). The data investigation was done by RBD Auger Scan 3.21 software. Differential scanning calorimetry (DSC) analysis was carried out on a STA409PC thermal analyzer (Netzsch, Germany) with 10°C/min heating rate in air. The phase analysis of NRs was identified with powder X-ray diffraction analysis using a D/max 2550 V diffractometer with Cu Kα radiation (λ = 1.54056 Å) (Rigaku, Tokyo, Japan). One typical sensor containing Ag-ZnO NRs was further visualized by scanning electron microscopy (SEM: JSM-6700F, 10 kV) after working for 100 days [54]. The surface morphology visualization of the final product was investigated with a field-emission scanning electron microscopy (FESEM, JEOL JSM-6500F). The electrodeposited NRs have cross-sectional SEM image with 100–300 nm diameter and 1–2 μm in length. TEM low magnification image confirmed that ZnO NRs are well aligned in nature. Selected area electron diffraction (SAED) indicates that NRs are single crystalline ZnO with wurtzite structure. Energy dispersive analysis con-

Instruments, Inc.) [42].

Gas Sensors

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 Tristar-3000 apparatus [38].

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

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

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

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,

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

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

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

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

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

another reference.

may be in the form of nanowires or nanorods [5].

their process gained a 90% progress rate.

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

with the device construction [73–75].

back and front surface of the ZnO pellets.

gas detection applications.

5.1 Gas detection from nanostructural materials

sensing.

71
