1. Introduction

The semiconductor metal oxides play the role of an encouraging contester for gas sensing employment. As a result, they have greater sensitivity for target gases due to easy synthesis techniques, being cheaper, and great compatibility with other techniques [1]. Nowadays, nanoarchitectures are being designed from SnO2, In2O3, ZnO, WO3, TiO2, CdO, TeO2, MoO3, CuO, and Fe2O3 with various dimensions and configurations. It was explored that surface morphology plays an important role in sensing applications [2, 3]. Semiconductor metal oxide nanostructures have various compositions and morphologies such as single crystals, one dimensional, and thin or thick film form due to utilization of interest and facility of synthesis techniques [4]. Recently, 1D nanoarchitectors have much attraction

for sensing due to their aspect ratio beside their thermal and chemical stabilities at various working situations [5, 6].

In nanotechnology and nanoscience, advancement in synthesis techniques for manufacturing 1D nanostructures has been an important target [7]. Various techniques have been adopted to fabricate 1D nanostructures for gas detection applications. These fabrication methods are sol-gel [7], ultrasonic irradiation [8], solid-state chemical reaction [9], electrospinning [10], molecular beam epitaxy [11], hydrothermal [12], thermal evaporation [13], molten-salt [14], anodization [15], RF sputtering [16], vapor-phase transport [17] chemical vapor deposition [18], carbothermal reduction [19], nanocarving [20], dry plasma etching [21], aerosol [22], and UV lithography. Various kinds of nanostructures can be fabricated with different morphologies that are directed by fabricating techniques and treatments.

Nanostructures grown by these techniques contain the shape of nanowires [19], nanobelts [16], nanotubes [15], nanoneedles [23], nanorods [5], nanofibers [12], nanoribbons [24], nanowhiskers [25], urchins [26], nanopushpins [27], fiber-mats [22], hierarchical dendrites [17], nanocarving [20], and lamellar [28]. The modifications in surface morphology result in sensing of various kinds of oxidizing and reducing gases like CO, NH3, NO2, H2, O2, H2S, LPG, xylene, propane, toluene, triethylamine, methanol, and acetone.

The sensitivity of a one-dimensional nanostructure sensor can increase with the improvement in surface morphology and bulk characteristics. These improvements can be made by doping with other elements or decorating nanoparticles on the surface of nanostructures. Sensors using such kinds of surface morphology and bulk characteristic improvements indicate significantly greater response as compared to that of unrestricted sensors.

The current article shows a detailed discussion of the contemporary investigation of the vigorous developments and adopting techniques for designing 1D SMO sensors. The sensor synthesize with nanostructures shows high sensitivity. This article's exemplary supposition investigates the 1D ZnO nanostructure sensor for ethanol detection process. Finally, conclusions explain the possible future evolution in 1D zinc oxide nanostructures for ethanol sensing.

### 2. Synthesis of ZnO nanostructures

The growth of ZnO nanostructures can be categories as follows: (a) wet chemical method, (b) solid-state synthesis, and (c) vapor-phase process. Wet chemical process consists of hydrothermal and ultrasonic irradiation growth in solution. However, ZnO nanostructures can be fabricated by solid-state route either through the solid-state chemical reaction or through the carbothermal reduction. The chemical vapor deposition process, molecular beam epitaxy (MBE), thermal evaporation, RF sputtering, vapor-phase transport, and aerosol are included in vapor-phase route. The processing particulars for synthesis of ZnO 1D nanostructure are illustrated in Table 1.

#### 2.1 Wet chemical process for the growth of one-dimensional nanostructures

#### 2.1.1 Hydrothermal process

Liwei Wang has synthesized ZnO NRs with hydrothermal technique in greater quantity (about 85%), in Zn(OH)4 <sup>2</sup>� solution and the presence of cetyltrimethylammonium bromide. CTAB act as a structure director, in the absence of any calcination technique [29].

Processing

59

Synthesis technique

 Starting material

Synthesis

temperature

 (°C)

Morphology

 Diameter of

Length of

Reference

ZnO

ZnO

nanostructure

nanostructure

route

Wet

Low-temperature

 process

(Zn(NO3)2�6H O), NaOH, CTAB

2

90°C for 15 h

Room tem. 400 for 3 h

Low tem.

NRs

90°C 3 h, 80°C for 12 h oC Flower-like

ZnO nanorods

ZnO NRs Nanorods

 45 nm

 1 μm

[34]

 150 nm

 4 μm

[33]

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

200–500 nm

 1.0–1.5 μm

 [32]

CuO-ZnO

NRs

ZnO capsules

300 nm NRs

 300 nm

ZnO nanorods

 90–200 nm

 1.7–2.1 μm

 [29] [30]

[31]

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

[32]

process

in autoclave

route

Hydrothermal

Hydrothermal

Wet chemical technique

Wet chemical low-

1.0239 g ZnCl2 (7.50 mmol), 3.006 g NaOH

(75.00 mmol),

25 mL deionized water

temperature

Two-step solution growth Solution growth technique

Hydrothermal

 technique

 ZnCO3,

LiNbO3

Zinc nitrate, polyethylenimine

ZnAc2�2H O,

2

Zn(NO3)2.6H

C6H12

10 ml Zn(Ac)2.2H O in 0.1 M methanol, 20 ml

NaOH in 0.5 M methanol, DI water

(K2SnO3�3H O, 95%), 0.75 g of urea

Zn(NO3)2�6H

C6H12

20 mM Zn(NO3)2 20 mM

hexamethylenetetramine

N4

O2

80°C deposition time was

ZnO NRs

 30–50 nm

—

[37]

16 h

90°C for 100 min, dried for

Well-dispersed

290–330 nm

 3.2–3.4 μm

 [38]

> ZnO nanorods

> 12 h at 60°C and annealed 1 h

at 500°C

2

2

150°C for 24 h for ZnO NRs

ZnO NRs

 2.8 nm, 2.5 nm 26 and 22 nm [36]

> and for ZnSnO3 3 h at 170°C

N4

O2

poly(ethylene

 glycol) 3 g of NaOH 120°C for 12 h. Dried 60°C

for 4 h

80°C

Ag-ZnO NRs,

—

 —

[31]

nigger-brown

Nanorods

 30–50 nm

 1.2 μm

 [35]

 in deionized water

hexamethylenetriamine,

 and

95°C

80°C for 12 h

CO2-ethanol,

 KOH

 process

ZnCl2 in NH4OH

 Ag/Ag

O2 3/ZnO NRs

 process

 CuO co-doped ZnO


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

for sensing due to their aspect ratio beside their thermal and chemical stabilities

In nanotechnology and nanoscience, advancement in synthesis techniques for manufacturing 1D nanostructures has been an important target [7]. Various techniques have been adopted to fabricate 1D nanostructures for gas detection applications. These fabrication methods are sol-gel [7], ultrasonic irradiation [8], solid-state chemical reaction [9], electrospinning [10], molecular beam epitaxy [11], hydrothermal [12], thermal evaporation [13], molten-salt [14], anodization [15], RF sputtering [16], vapor-phase transport [17] chemical vapor deposition [18], carbothermal reduction [19], nanocarving [20], dry plasma etching [21], aerosol [22], and UV lithography. Various kinds of nanostructures can be fabricated with different morphologies that are directed by fabricating techniques

Nanostructures grown by these techniques contain the shape of nanowires [19], nanobelts [16], nanotubes [15], nanoneedles [23], nanorods [5], nanofibers [12], nanoribbons [24], nanowhiskers [25], urchins [26], nanopushpins [27], fiber-mats [22], hierarchical dendrites [17], nanocarving [20], and lamellar [28]. The modifications in surface morphology result in sensing of various kinds of oxidizing and reducing gases like CO, NH3, NO2, H2, O2, H2S, LPG, xylene, propane, toluene,

The sensitivity of a one-dimensional nanostructure sensor can increase with the improvement in surface morphology and bulk characteristics. These improvements can be made by doping with other elements or decorating nanoparticles on the surface of nanostructures. Sensors using such kinds of surface morphology and bulk characteristic improvements indicate significantly greater response as compared to

The current article shows a detailed discussion of the contemporary investigation of the vigorous developments and adopting techniques for designing 1D SMO sensors. The sensor synthesize with nanostructures shows high sensitivity. This article's exemplary supposition investigates the 1D ZnO nanostructure sensor for ethanol detection process. Finally, conclusions explain the possible future evolution

The growth of ZnO nanostructures can be categories as follows: (a) wet chemical method, (b) solid-state synthesis, and (c) vapor-phase process. Wet chemical process consists of hydrothermal and ultrasonic irradiation growth in solution. However, ZnO nanostructures can be fabricated by solid-state route either through the solid-state chemical reaction or through the carbothermal reduction. The chemical vapor deposition process, molecular beam epitaxy (MBE), thermal evaporation, RF sputtering, vapor-phase transport, and aerosol are included in vapor-phase route. The processing

particulars for synthesis of ZnO 1D nanostructure are illustrated in Table 1.

2.1 Wet chemical process for the growth of one-dimensional nanostructures

Liwei Wang has synthesized ZnO NRs with hydrothermal technique in

methylammonium bromide. CTAB act as a structure director, in the absence of any

<sup>2</sup>� solution and the presence of cetyltri-

at various working situations [5, 6].

triethylamine, methanol, and acetone.

in 1D zinc oxide nanostructures for ethanol sensing.

2. Synthesis of ZnO nanostructures

greater quantity (about 85%), in Zn(OH)4

that of unrestricted sensors.

2.1.1 Hydrothermal process

calcination technique [29].

58

and treatments.

Gas Sensors


Processing

61

Synthesis technique

 Starting material

Synthesis

temperature

 (°C)

Morphology

 Diameter of

Length of

Reference

ZnO

ZnO

nanostructure

nanostructure

route

Seeding by thermolysis-assisted

sol. tech. Solvothermal

Solid-state

technique

Vaporphase

Carbothermal

 reduction

 0.3 g Zn powder and 0.6 g active carbon

 900°C at a rate of 25oC/min

Nest-like ZnO —

 —

[51]

for 2 h

 900°C 600°C

NRs

Average

11.8 μm, 6.5,

[53]

diameter

and 3.5 μm

> 400 nm,

550 nm

NWs

50–80 nm thick Several micrometers

[52]

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

technique

Continuous

growth

Table 1. Details of different growth techniques of ZnO 1D

nanoarchitectures.

 two-step vapor

(CH3)4Sn, DI water, IPA, PDMS substrate

ZnO powder

Zn(CH3COO)2�2H

 O, methanol, KOH

2

70°C. 3–5 days

NRs

15 nm

 50–120 nm

 [50]

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

 aqu.

RF-sputtering,

0.01 mol C6H12N4, in 400 ml DI water

 85°C

ZnO nanorods

 8–160 nm thick

—

[30]

seed layer

Gas Sensors



Details of different growth techniques of ZnO 1D nanoarchitectures.

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

Processing

60

Synthesis technique

 Starting material

Synthesis

temperature

 (°C)

Morphology

 Diameter of

Length of

Reference

Gas Sensors

ZnO

ZnO

nanostructure

nanostructure

route

Sol-gel

Zn(CH3COO)2�2H

Zn(CH3COO)2,

zinc acetate dehydrate, (CH2)6N4

> One-step

method

Hydrothermal

Ultrasonic irradiation in

aqueous solution

DC magnetron sputtering

 ZnO (0, 1, 10, and 25 at. wt%) ZnO, MoO3, Au-

IDE, acetone

Metallic zinc foil and formamide

ITO glass, 7

3 h),170 nm thick seed layer Zn(NO3) 6H O (10 mM), (C

CoSO4�7H O, 0 2 Zn(NO3)2(H O) 2 6, NH3

Microwave

 irradiation

 Zinc acetate dihydrate, NaOH

(CH3COOH)2�2H

 O, HMTA, CTAB

2

–0.5 mM

2

6H12

N4,10 mM),

90°C for 15–30 min

80°C for 3 h Dried at 60°C Calcined 600°

C, 2 h

120°C for 12 h in autoclave

ZnO

—

 —

[47]

nanocrystals

ZnO flower-

200–300 nm

 1.5 μm

 [48]

> like

 Hexagonal

17 nm

 90 nm

 [49]

ZnO

nanosheets

 ZnO nanorods

 100–300 nm

 1–3 μm

 [46]

Solution method

Ω/sq., CBD (CBD condition: 95°C,

400°C for 1 h

 solution

 55°C

Hexagonalshaped ZnO

400 nm

 2 μm

[44]

nanorods

ZnO nanorods

 55 nm

 1.5 μm

 [45]

 synthesis

water CTAB Alumina substrate,

Zn(NO3)2�6H O, (CH 2

2)6N4 —

Zn(NO3)2.6H O sodium hydroxide deionized

2

solvothermal

20 mM Zn(NO3)2 and 20 mM HMTA

 2H O 2-propanol

2

 C

4H11NO2,

75°C for 30 min

90°C for 100 min

120°C for 12h

ZnO nanorods

Nanorod

50 nm

 500 nm

 [11]

(vertically

aligned)

Nanostructures

 —

 —

[43]

Annealing in ambient air for

1 h at 773 K

 80–150 nm

—

[42]

Nanorods

 220 nm

—

[41]

Hexagonal NRs 60–70 nm

O2

95°C 50 ml autoclave

 Nanorods

 51 and 33 nm 262, 748, and

470nm

 -

[40]

[39]

Semiconductor metal-doped transition nanomaterials have a number of applications. The hydrothermal growth technique is cost-effective and environmentfriendly. The nanostructures of CuO doped with ZnO synthesized at room temperature by hydrothermal technique need great efforts. The properties of CuO doped with ZnO nanorods were measured by various spectroscopy techniques, and sensing was executed by I-V analysis. Aqueous ammonia was used as analytic gas as reported by M. Rahman et al. CuO nanorods doped with ZnO sensor show good stability, sensitivity, and reproducibility. Sensor based on signal/noise ratio has sensitivity of 1.549 0.10 <sup>μ</sup>A cm<sup>2</sup> Mm<sup>1</sup> . Sensor applications depend on the transition metal coated with ZnO nanomaterial [30].

noise ratio having the value of 7.917 Μa cm<sup>2</sup> mmol L<sup>2</sup> with a detection range of

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

lent semiconductor nanomaterial for sensing acetone with silver electrodes. The detector exhibits high sensitivity, reproducibility, and stability. The calibration graph is linear and exhibits high sensitivity and small detection range in large

Here, we investigate hydrothermal growth of Ag/Ag2O3/ZnO NRs that exhibit high crystalline structure. The characterization confirms that composite NRs have 300 nm cross-section areas. The composite NRs have absorption edge at 375 nm that exhibited that they are optically active. The sensor shows high sensitivity and small detection range. Hence, NRs can be used as redox mediator for efficient phenyl-

Good-quality crystalline ZnO NPs were prepared by hydrothermal process using aqueous solution of zinc chloride and ammonium hydroxide. Nanoparticles have wurtzite geometry with an approximate size of 50 60 nm. The composition quality of the as-prepared NPs exhibited 1:1 stoichiometry of Zn and O2, which was confirmed from spectroscopy analysis. The optical characterization of ZnO NPs exhibits that they are used as photocatalyst for the degradation of acridine-orange and for the detection of acetone. The constructed sensor shows high sensitivity of

ZnO NRs in vertical direction were synthesized on flexible polyimide films by a

ZnO nanorod arrays with various aspect ratios and characteristics relevant to tailored defect are prepared and used to synthesize gas sensors. The ZnO gas sensors are of three different types synthesized by sol-gel process and successive aqueous chemical growth (ACG). According to Shi et al. and Vayssieres et al., ACG solutions used in the current research work are two types: Zn salt/KOH and Zn

An accomplished photochemical method was used for the synthesis of Ag NPs decorated on ZnO nanorods for investigation of gas sensing. ZnO NRs decorated

NRs with and without Co doping were synthesized in 10 mM solution of zinc nitrate hexahydrate Zn(NO3)6H2O, hexamethylenetetramine, and 0.5 mM cobalt sulfate hydrate (CoSO47H2O) in water at 90°C for 15–30 min. The sensor showed high response and sensitivity as NRs fabricated by electrodeposition process [44]. To fabricate ZnO nanorods by solvothermal technique, prepare a 1.12 M solution

of zinc acetate dihydrate in methanol (40 mL) under vigorous stirring at 70°C. Particularly, at 70°C, add 21.33 mL solution of KOH (21.33 mL) dropwise in methanol. After 2 h of vigorous stirring of the mixture, transfer it to 100 mL autoclaves, which were allowed to react at 70°C for 3–5 days and then cooled down naturally.

1D ZnO NRs were grown on ITO-coated glass (AUO Co., Ltd.). The ITO glass

samples were thermally treated at 400°C for 1 h in an electric furnace after deposition. Ethanol of analytical grade and deionized water with 2.61 μs/cm and pH 6.7

has 7 Ω/sq. resistance in sequential chemical bath deposition. A seeding layer was deposited with a thickness of 170 nm at 25°C by the process of dip coating in the first step and then followed by the process of CBD at 95°C for 3 h with the synthesis of ZnO nanorods in 0.05 M precursor solution of ZnO. The ZnO

different spectroscopic techniques. NRs were calcined at 400<sup>o</sup>

0.14065 μA cm<sup>2</sup> mM<sup>1</sup> and small detection range in 10 s [32].

with Ag increased long-term stability and response time [54].

thermolysis-assisted chemical solution method [42].

A reliable technique for detecting dissolved acetone by doped nanomaterials was investigated by Mohammed M. Rahman et al. ZnO NRs co-doped with Co3O4 were fabricated by solvothermal process. The as-fabricated NRs were characterized for

C that shows excel-

<sup>71</sup> 0.5 <sup>μ</sup>mol L<sup>1</sup> [45].

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

concentration of acetone [40].

hydrazine detector [33].

salt/HMTA [41].

Harvest the resultant product [35].

were used in the experiment [49].

63

ZnO nanoparticles were synthesized hydrothermally from urea and zinc chloride and characterized by various spectroscopic techniques. The ZnO NPs range in size from 180 to 130 nm with hexagonal structure. The photocatalyst analysis of ZnO nanoparticles was determined with the degradation of methylene blue. ZnO nanostructures were possessed by high photocatalyst value as compared to TiO2-UV100. Moreover, the sensing characteristics of the detector were determined with methanol by I-V process at room temperature. It was investigated that sensor shows sensitivity in the range of 0.9554 μA cm<sup>2</sup> mM<sup>1</sup> at room temperature [39].

ZnO nanocapsules have been synthesized hydrothermally. The structural morphology and characteristics were analyzed with the help of different spectroscopy techniques. The sensing was measured with I-V method by using chloroform as the detecting gas. Sensor shows high sensitivity, small detection limit, and high linear dynamic limit with fine linearity in a small response time. Moreover, the photocatalytic response was calculated with the degradation of acridine orange [31].

Nanoparticles of ZnO-CeO2 were grown by hydrothermal process. These nanostructures were used for the construction of sensor and photocatalyst by elimination of environmental pollution. These NOs have diameter approximately from 50 10 nm. The composition of NPs was measured by EDS spectroscopy, and UV-visible spectrum investigates the optical characteristics. The photocatalytic degradation of acridine orange and methylene blue dyes has been carried out using ZnO-CeO2 NPs. The critical routine of ZnO-CeO2 NPs for ethanol detector shows high sensitivity and small detection limit in shorter time [46].

The nanostructures of ZnO-CeO2 were grown by hydrothermal technique. These nanostructures have extended shaped CeO2 NPs having diameters of 40–90 nm. Photocatalytic activity of CeO2 co-doped ZnO structure was measured with degradation of acridine orange and methylene blue. Sensor fabricated by ZnO-CeO2 nanostructures shows high sensitivity in 10 s. The chemical sensor used ethanol as the sensing gas using I-V procedure [50].

ZnO NR sensor for ammonia detection has been synthesized by wet chemical technique at cost-effective low temperature. The as-synthesized sensor exhibited high sensitivity of 5.538 mA cm<sup>2</sup> mM<sup>1</sup> , in small detection range of 0.11 mM, with linear-dynamic limit from 0.5 mM, having high linearity of 0.7102 in 10.0 s response time. In addition, sensor performs good for the degradation of acridineorange, methylene-blue, and amido black. Spectroscopy analysis exhibited that ZnO NRs have wurtzite crystalline structure with diameter of 58.61 nm [50].

The calcined AgO co-doped ZnO NPs were synthesized by hydrothermal process using alkaline medium as reducing agent. The as-prepared NPs were analyzed by different spectroscopy techniques. Sensor was fabricated on a microchip for methanol sensing that exhibited high response. Careful observation exhibits that microchip sensors have high value of sensitivity, reliability, small volume reproducibility with ease of integration, high stability, and good response. The calibration graph is linear over methanol concentration. The sensitivity was measured at 3 at signal-tonoise ratio having the value of 7.917 Μa cm<sup>2</sup> mmol L<sup>2</sup> with a detection range of <sup>71</sup> 0.5 <sup>μ</sup>mol L<sup>1</sup> [45].

A reliable technique for detecting dissolved acetone by doped nanomaterials was investigated by Mohammed M. Rahman et al. ZnO NRs co-doped with Co3O4 were fabricated by solvothermal process. The as-fabricated NRs were characterized for different spectroscopic techniques. NRs were calcined at 400<sup>o</sup> C that shows excellent semiconductor nanomaterial for sensing acetone with silver electrodes. The detector exhibits high sensitivity, reproducibility, and stability. The calibration graph is linear and exhibits high sensitivity and small detection range in large concentration of acetone [40].

Here, we investigate hydrothermal growth of Ag/Ag2O3/ZnO NRs that exhibit high crystalline structure. The characterization confirms that composite NRs have 300 nm cross-section areas. The composite NRs have absorption edge at 375 nm that exhibited that they are optically active. The sensor shows high sensitivity and small detection range. Hence, NRs can be used as redox mediator for efficient phenylhydrazine detector [33].

Good-quality crystalline ZnO NPs were prepared by hydrothermal process using aqueous solution of zinc chloride and ammonium hydroxide. Nanoparticles have wurtzite geometry with an approximate size of 50 60 nm. The composition quality of the as-prepared NPs exhibited 1:1 stoichiometry of Zn and O2, which was confirmed from spectroscopy analysis. The optical characterization of ZnO NPs exhibits that they are used as photocatalyst for the degradation of acridine-orange and for the detection of acetone. The constructed sensor shows high sensitivity of 0.14065 μA cm<sup>2</sup> mM<sup>1</sup> and small detection range in 10 s [32].

ZnO NRs in vertical direction were synthesized on flexible polyimide films by a thermolysis-assisted chemical solution method [42].

ZnO nanorod arrays with various aspect ratios and characteristics relevant to tailored defect are prepared and used to synthesize gas sensors. The ZnO gas sensors are of three different types synthesized by sol-gel process and successive aqueous chemical growth (ACG). According to Shi et al. and Vayssieres et al., ACG solutions used in the current research work are two types: Zn salt/KOH and Zn salt/HMTA [41].

An accomplished photochemical method was used for the synthesis of Ag NPs decorated on ZnO nanorods for investigation of gas sensing. ZnO NRs decorated with Ag increased long-term stability and response time [54].

NRs with and without Co doping were synthesized in 10 mM solution of zinc nitrate hexahydrate Zn(NO3)6H2O, hexamethylenetetramine, and 0.5 mM cobalt sulfate hydrate (CoSO47H2O) in water at 90°C for 15–30 min. The sensor showed high response and sensitivity as NRs fabricated by electrodeposition process [44].

To fabricate ZnO nanorods by solvothermal technique, prepare a 1.12 M solution of zinc acetate dihydrate in methanol (40 mL) under vigorous stirring at 70°C. Particularly, at 70°C, add 21.33 mL solution of KOH (21.33 mL) dropwise in methanol. After 2 h of vigorous stirring of the mixture, transfer it to 100 mL autoclaves, which were allowed to react at 70°C for 3–5 days and then cooled down naturally. Harvest the resultant product [35].

1D ZnO NRs were grown on ITO-coated glass (AUO Co., Ltd.). The ITO glass has 7 Ω/sq. resistance in sequential chemical bath deposition. A seeding layer was deposited with a thickness of 170 nm at 25°C by the process of dip coating in the first step and then followed by the process of CBD at 95°C for 3 h with the synthesis of ZnO nanorods in 0.05 M precursor solution of ZnO. The ZnO samples were thermally treated at 400°C for 1 h in an electric furnace after deposition. Ethanol of analytical grade and deionized water with 2.61 μs/cm and pH 6.7 were used in the experiment [49].

Semiconductor metal-doped transition nanomaterials have a number of applica-

. Sensor applications depend on the

, in small detection range of 0.11 mM, with

tions. The hydrothermal growth technique is cost-effective and environmentfriendly. The nanostructures of CuO doped with ZnO synthesized at room temperature by hydrothermal technique need great efforts. The properties of CuO doped with ZnO nanorods were measured by various spectroscopy techniques, and sensing was executed by I-V analysis. Aqueous ammonia was used as analytic gas as reported by M. Rahman et al. CuO nanorods doped with ZnO sensor show good stability, sensitivity, and reproducibility. Sensor based on signal/noise ratio has

ZnO nanoparticles were synthesized hydrothermally from urea and zinc chloride and characterized by various spectroscopic techniques. The ZnO NPs range in size from 180 to 130 nm with hexagonal structure. The photocatalyst analysis of ZnO nanoparticles was determined with the degradation of methylene blue. ZnO nanostructures were possessed by high photocatalyst value as compared to TiO2-UV100. Moreover, the sensing characteristics of the detector were determined with methanol by I-V process at room temperature. It was investigated that sensor shows sensitivity in the range of 0.9554 μA cm<sup>2</sup> mM<sup>1</sup> at room

ZnO nanocapsules have been synthesized hydrothermally. The structural morphology and characteristics were analyzed with the help of different spectroscopy techniques. The sensing was measured with I-V method by using chloroform as the detecting gas. Sensor shows high sensitivity, small detection limit, and high linear

photocatalytic response was calculated with the degradation of acridine orange [31]. Nanoparticles of ZnO-CeO2 were grown by hydrothermal process. These nanostructures were used for the construction of sensor and photocatalyst by elimination of environmental pollution. These NOs have diameter approximately from 50 10 nm. The composition of NPs was measured by EDS spectroscopy, and UV-visible spectrum investigates the optical characteristics. The photocatalytic degradation of acridine orange and methylene blue dyes has been carried out using ZnO-CeO2 NPs. The critical routine of ZnO-CeO2 NPs for ethanol detector shows

The nanostructures of ZnO-CeO2 were grown by hydrothermal technique. These nanostructures have extended shaped CeO2 NPs having diameters of 40–90 nm. Photocatalytic activity of CeO2 co-doped ZnO structure was measured with degradation of acridine orange and methylene blue. Sensor fabricated by ZnO-CeO2 nanostructures shows high sensitivity in 10 s. The chemical sensor used ethanol as

ZnO NR sensor for ammonia detection has been synthesized by wet chemical technique at cost-effective low temperature. The as-synthesized sensor exhibited

The calcined AgO co-doped ZnO NPs were synthesized by hydrothermal process using alkaline medium as reducing agent. The as-prepared NPs were analyzed by different spectroscopy techniques. Sensor was fabricated on a microchip for methanol sensing that exhibited high response. Careful observation exhibits that microchip sensors have high value of sensitivity, reliability, small volume reproducibility with ease of integration, high stability, and good response. The calibration graph is linear over methanol concentration. The sensitivity was measured at 3 at signal-to-

linear-dynamic limit from 0.5 mM, having high linearity of 0.7102 in 10.0 s response time. In addition, sensor performs good for the degradation of acridineorange, methylene-blue, and amido black. Spectroscopy analysis exhibited that ZnO

NRs have wurtzite crystalline structure with diameter of 58.61 nm [50].

dynamic limit with fine linearity in a small response time. Moreover, the

high sensitivity and small detection limit in shorter time [46].

the sensing gas using I-V procedure [50].

high sensitivity of 5.538 mA cm<sup>2</sup> mM<sup>1</sup>

62

sensitivity of 1.549 0.10 <sup>μ</sup>A cm<sup>2</sup> Mm<sup>1</sup>

temperature [39].

Gas Sensors

transition metal coated with ZnO nanomaterial [30].

ZnO nanorods were prepared from HMT solution on p-Si substrate using oxidation furnace at 1000°C for drying 30 min following 2 h wet oxidation process. The seed layer for ZnO nanocrystal was prepared with zinc acetate dihydrate (Zn(CH3COO)22H2O) in 2-propanol by sol-gel process. Then, 3 ml diethanolamine (C4H11NO2) was added dropwise to clear the solution and aged for 24 h. The samples were spin coated and annealed at 450°C for 1 h. A 50 ml solution in deionized water (18.2 MΩ cm) of zinc acetate dihydrate and (CH2)6N4 in 1:3 mass ratio was prepared. Then, the substrate was placed into the nucleation solution at 75°C for 30 min [36].

The ZnO seeding layer will act as nucleation site for the fabrication of ZnO NRs from hydrothermal process. An equimolar growth solution of zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and C6H12N4 was prepared. The transducers were washed ultrasonically in acetone and dried in nitrogen atmosphere. Then, the transducer was kept inverted on Teflon holder and attached to a glass rod before inserting into the reaction bottle. The bottle was placed in an oven for 16 h at 80°C. Finally, the substrate was washed thoroughly to remove impurities and dried in N2 stream at a

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

A solution of 1 mM zinc acetate dihydrate (Zn(CH3COOH)2�2H2O),

2.2.1 Preparation of seed layer and ZnO nanorod arrays

obtained and rinsed with distilled water [34].

electrodes were printed [57].

65

2.3 Solid-state processing route (microwave irradiation)

for 12 h at 60°C and annealed in air for 1 h at 500°C [58].

0.5 mM hexamethylenetetramine, and 0.05 g cetyltrimethylammonium bromide (CTAB) in 40 mL deionized water was prepared after vigorously stirring for 1 h. Then, the mixture was transferred in autoclave and placed under heat treatment for 12 h at 120°C. After centrifugation, the white precipitates were harvested, rinsed with ethanol and deionized water. Finally, the product was dried in air atmosphere

The ZnO nanorod array was fabricated by dissolving 10 ml Zn(Ac)2�2H2O in 0.1 M methanol and 20 ml NaOH in 0.5 M methanol solution. After filtration, the solution was transferred to a 50 ml autoclave and heat treated for 24 h at 150°C. The precipitates were collected and washed. For hydrothermal growth of ZnSnO3 nanorods, 0.06 g of ZnO nanorods was dissolved by ultrasonication (Power, 500 mW) in 32 ml of water-alcohol (38 vol% alcohol) mixture. Then, 0.115 g of potassium stannate trihydrate (K2SnO3�3H2O, 95%) and 0.75 g of urea were transferred to the above mixture. After vigorous stirring for 5–10 min, the solvent was shifted into a 50 ml autoclave and heated for 3 h at 170°C. A white product was

A 0.1 M solution of zinc acetate dehydrates was prepared and vigorously stirred for 10 min. To obtain 0, 2, 5, and 10 wt% samaria, a suitable quantity of samarium nitrate was added during magnetically stirring. Then, guanidinium carbonate (0.1 M) solution was dropped to the mixture. Consequently, 10 min latter, 4 M solution of NaOH also was added dropwise to attain a clear solution with pH 12. Lastly, the microwave irradiation (with 75% power) was exposed to the mixture for 2 min. For irradiation, a microwave oven was used with a power of 800 W (Concave Reflex Systems (C.R.S.), KOR-63A5). The white precipitates were separated, rinsed, dried under vacuum at 60°C, and calcined for 2 h at 600°C in air [56]. Interdigital electrodes (IDTs) were placed on the upper surface of alumina substrate (96%), with line/space of 30 μm. Actually, such kind of line-printing is not feasible with thick film method. By a laser printer of gold film, the interdigital

A 20 mM nutrient solution (equal molar solution) of Zn(NO3)2 and HMTA in distilled water was prepared, and 4:1 volume ratio of the anhydrous ethanol was added. Precursors were formulated by dispersing in their relative solvents. The container with precursors was put in an oven for 100 min at 90°C. After cooling, the white sludge was collected and cleaned using centrifuge and rinsing water. Finally, the sludge was dried

flow of 200 sccm [51].

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

at 60°C [52].

2.2 Sol-gel process

ZnO NRs were fabricated from ZnCO3 fibers that were formed in a solution of CO2-ethanol, by consecutive treatment in KOH solution. The ZnO NRs were single crystal with a diameter of 150 nm and length of 4 μm. It was observed that ethanol sensing was done at 250°C [48].

ZnO nanorods having a flower-like shape were fabricated at 90°C by the hydrothermal process in the absence of any surfactant, organic solvent, or toxic regent. In a particular growth, solution A was formed by dissolving 7.50 mmol ZnCl2 in 25 mL deionized water under stirring. Adopting the similar technique, 75.00 mmol NaOH was dissolved in 25 ml deionized water consequence in solution B. Finally, solution A was added into B dropwise under stirring. The resultant solution was placed at 90°C for 3 h, harvesting the white precipitate rinsed with deionized water and drying them in vacuum for 12 h at 80°C [38].

ZnO nanorods were fabricated by dissolving 2.97 g Zn(NO3)26H2O and 4.00 g NaOH (molar ratio of Zn2+/OH by1:10) in 200 ml DI water. Consequently, 5.00 g CTAB was added under vigorous stirring for 1 h. The solution was shifted to a Teflon-lined steel autoclave and kept at 120°C for 12 h and then allowed to cool down naturally. Then, white precipitates were collected, washed, and dried overnight at 60°C. At a pressure of 6.0 MPa, ZnO NRs were used to design a sensor. The pellets were 3.0 mm thick and have an area of 5.3 cm<sup>2</sup> . On the surface of pellets, electrodes of 2.0 cm<sup>2</sup> were deposited by silver paste [47].

A 20 mM nutrient solution of Zn(NO3)2 and hexamethylenetetramine (HMTA) was prepared. The used precursors are such a kind that dissolved in necessary solvents. After dissolving Zn(NO3)2 and HMTA precursor solutions, the beaker was covered. Then, it was put in an oven for 100 min at 90°C. After cooling down naturally, the white precipitates were collected by centrifuging and rinsed and dried at 60°C for 12 h. Calcinations were done for 1 h at 500°C in the presence of air. The samples prepared with 0, 10, 20, 30, 40, and 50 vol% ethanol solvent were numbered a0, a10, a20, a30, a40, and a50, respectively [55].

To fabricate one-dimensional multifaceted ZnO nanostructures, a new strategy has been introduced that consists of two-phase solution process. Through these techniques, nanowires, nanorod arrays (NAs), nanotubes, nanorod hollow spheres (NHSs), nanoribbons, and nanonetworks were grown at low temperature without any catalysts, templates, or precursors [53].

ZnO nanostructures with different morphologies have been grown from zinc foil (1 1 cm<sup>2</sup> ) and substrates were cleaned by ultrasonication process in ethanol. The substrates were suspended vertically to the bottom of a vial in formamide solution 3 ml of 15%. The vial with substrate was sealed and put inside an oven with fixed temperature at 55°C. The samples were washed thoroughly with ethanol and dried. Time-dependent experiments estimate the illumination transformation process as well as reaction time of ZnO nanodisks from NRs [43].

Filtered cathodic vacuum arc (FCVA) mechanism was introduced to deposit ZnO layer on the surface of alumina substrates decorated with gold IDTs. The substrates were loaded in the holder with varied temperature heater. After completion of deposition, the substrate was carried out for analysis from the chamber.

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

The ZnO seeding layer will act as nucleation site for the fabrication of ZnO NRs from hydrothermal process. An equimolar growth solution of zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and C6H12N4 was prepared. The transducers were washed ultrasonically in acetone and dried in nitrogen atmosphere. Then, the transducer was kept inverted on Teflon holder and attached to a glass rod before inserting into the reaction bottle. The bottle was placed in an oven for 16 h at 80°C. Finally, the substrate was washed thoroughly to remove impurities and dried in N2 stream at a flow of 200 sccm [51].

A solution of 1 mM zinc acetate dihydrate (Zn(CH3COOH)2�2H2O),

0.5 mM hexamethylenetetramine, and 0.05 g cetyltrimethylammonium bromide (CTAB) in 40 mL deionized water was prepared after vigorously stirring for 1 h. Then, the mixture was transferred in autoclave and placed under heat treatment for 12 h at 120°C. After centrifugation, the white precipitates were harvested, rinsed with ethanol and deionized water. Finally, the product was dried in air atmosphere at 60°C [52].

#### 2.2 Sol-gel process

ZnO nanorods were prepared from HMT solution on p-Si substrate using oxidation furnace at 1000°C for drying 30 min following 2 h wet oxidation process. The seed layer for ZnO nanocrystal was prepared with zinc acetate dihydrate (Zn(CH3COO)22H2O) in 2-propanol by sol-gel process. Then, 3 ml diethanolamine (C4H11NO2) was added dropwise to clear the solution and aged for 24 h. The samples were spin coated and annealed at 450°C for 1 h. A 50 ml solution in

deionized water (18.2 MΩ cm) of zinc acetate dihydrate and (CH2)6N4 in 1:3 mass ratio was prepared. Then, the substrate was placed into the nucleation solution at

ZnO NRs were fabricated from ZnCO3 fibers that were formed in a solution of CO2-ethanol, by consecutive treatment in KOH solution. The ZnO NRs were single crystal with a diameter of 150 nm and length of 4 μm. It was observed that ethanol

ZnO nanorods having a flower-like shape were fabricated at 90°C by the hydrothermal process in the absence of any surfactant, organic solvent, or toxic regent. In a particular growth, solution A was formed by dissolving 7.50 mmol ZnCl2 in 25 mL deionized water under stirring. Adopting the similar technique, 75.00 mmol NaOH was dissolved in 25 ml deionized water consequence in solution B. Finally, solution A was added into B dropwise under stirring. The resultant solution was placed at 90°C for 3 h, harvesting the white precipitate rinsed with deionized water and

ZnO nanorods were fabricated by dissolving 2.97 g Zn(NO3)26H2O and 4.00 g NaOH (molar ratio of Zn2+/OH by1:10) in 200 ml DI water. Consequently, 5.00 g CTAB was added under vigorous stirring for 1 h. The solution was shifted to a Teflon-lined steel autoclave and kept at 120°C for 12 h and then allowed to cool down naturally. Then, white precipitates were collected, washed, and dried overnight at 60°C. At a pressure of 6.0 MPa, ZnO NRs were used to design a sensor.

A 20 mM nutrient solution of Zn(NO3)2 and hexamethylenetetramine (HMTA)

To fabricate one-dimensional multifaceted ZnO nanostructures, a new strategy has been introduced that consists of two-phase solution process. Through these techniques, nanowires, nanorod arrays (NAs), nanotubes, nanorod hollow spheres (NHSs), nanoribbons, and nanonetworks were grown at low temperature without

ZnO nanostructures with different morphologies have been grown from zinc foil

substrates were suspended vertically to the bottom of a vial in formamide solution 3 ml of 15%. The vial with substrate was sealed and put inside an oven with fixed temperature at 55°C. The samples were washed thoroughly with ethanol and dried. Time-dependent experiments estimate the illumination transformation process as

Filtered cathodic vacuum arc (FCVA) mechanism was introduced to deposit ZnO layer on the surface of alumina substrates decorated with gold IDTs. The substrates were loaded in the holder with varied temperature heater. After completion of deposition, the substrate was carried out for analysis from the chamber.

) and substrates were cleaned by ultrasonication process in ethanol. The

was prepared. The used precursors are such a kind that dissolved in necessary solvents. After dissolving Zn(NO3)2 and HMTA precursor solutions, the beaker was covered. Then, it was put in an oven for 100 min at 90°C. After cooling down naturally, the white precipitates were collected by centrifuging and rinsed and dried at 60°C for 12 h. Calcinations were done for 1 h at 500°C in the presence of air. The samples prepared with 0, 10, 20, 30, 40, and 50 vol% ethanol solvent were

. On the surface of pellets,

75°C for 30 min [36].

Gas Sensors

sensing was done at 250°C [48].

drying them in vacuum for 12 h at 80°C [38].

The pellets were 3.0 mm thick and have an area of 5.3 cm<sup>2</sup>

electrodes of 2.0 cm<sup>2</sup> were deposited by silver paste [47].

numbered a0, a10, a20, a30, a40, and a50, respectively [55].

well as reaction time of ZnO nanodisks from NRs [43].

any catalysts, templates, or precursors [53].

(1 1 cm<sup>2</sup>

64

#### 2.2.1 Preparation of seed layer and ZnO nanorod arrays

The ZnO nanorod array was fabricated by dissolving 10 ml Zn(Ac)2�2H2O in 0.1 M methanol and 20 ml NaOH in 0.5 M methanol solution. After filtration, the solution was transferred to a 50 ml autoclave and heat treated for 24 h at 150°C. The precipitates were collected and washed. For hydrothermal growth of ZnSnO3 nanorods, 0.06 g of ZnO nanorods was dissolved by ultrasonication (Power, 500 mW) in 32 ml of water-alcohol (38 vol% alcohol) mixture. Then, 0.115 g of potassium stannate trihydrate (K2SnO3�3H2O, 95%) and 0.75 g of urea were transferred to the above mixture. After vigorous stirring for 5–10 min, the solvent was shifted into a 50 ml autoclave and heated for 3 h at 170°C. A white product was obtained and rinsed with distilled water [34].

#### 2.3 Solid-state processing route (microwave irradiation)

A 0.1 M solution of zinc acetate dehydrates was prepared and vigorously stirred for 10 min. To obtain 0, 2, 5, and 10 wt% samaria, a suitable quantity of samarium nitrate was added during magnetically stirring. Then, guanidinium carbonate (0.1 M) solution was dropped to the mixture. Consequently, 10 min latter, 4 M solution of NaOH also was added dropwise to attain a clear solution with pH 12. Lastly, the microwave irradiation (with 75% power) was exposed to the mixture for 2 min. For irradiation, a microwave oven was used with a power of 800 W (Concave Reflex Systems (C.R.S.), KOR-63A5). The white precipitates were separated, rinsed, dried under vacuum at 60°C, and calcined for 2 h at 600°C in air [56].

Interdigital electrodes (IDTs) were placed on the upper surface of alumina substrate (96%), with line/space of 30 μm. Actually, such kind of line-printing is not feasible with thick film method. By a laser printer of gold film, the interdigital electrodes were printed [57].

A 20 mM nutrient solution (equal molar solution) of Zn(NO3)2 and HMTA in distilled water was prepared, and 4:1 volume ratio of the anhydrous ethanol was added. Precursors were formulated by dispersing in their relative solvents. The container with precursors was put in an oven for 100 min at 90°C. After cooling, the white sludge was collected and cleaned using centrifuge and rinsing water. Finally, the sludge was dried for 12 h at 60°C and annealed in air for 1 h at 500°C [58].

## 2.4 Vapor-phase route (pulsed laser deposition)

ZnO nanorods were grown onto the long-period gratings (LPGs), followed by a two-step process. They contain pulsed laser deposition (Zn layer onto optical fiber), followed by the growth of ZnO nanostructures in aqueous solution. Zinc precursor layer was deposited by a laser beam (248 nm KrF excimer) that was focused on target (Zn target with 99.9% purity) to produce fluence (1.3 J/cm<sup>2</sup> ). The evaporated material was deposited on the optical fibers located at 6 cm apart from the target. According to identical conditions, the deposition on substrate has resulted in Zn layer nearly 40 mm thick, when exposed with 2000 laser pulses [59].

2.7 Surface acoustic wave

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

washed, and dried at room temperature [65].

copy for structural and morphological analysis.

surface of wafer.

3. Characterization

3.1 X-ray diffraction

90–200 nm.

67

3.2 Scanning electron microscopy

3.3 Transmission electron microscopy

The ZnO nanorod sensor was designed on Y-cut LiNbO3 substrate for ethanol sensing. The performance of SAW is excellent due to electromechanical coupling coefficient (k2 = 4.5%) offered by LiNbO3 and small attenuation at high temperature. The interdigital transducers (IDTs) have a gap of 12 mm delay line, and each contains 50 pairs of electrodes. The width and the spacing inside the adjacent IDT electrodes are adjusted 15 μm. The IDT electrodes have length of 3 mm. The propagation of SAW elongated in x-direction of the crystal as IDTs is oriented on the

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

Deposit a photoresistive layer by spin coated process and decorate it with window covering delay line for the construction of sensor. ZnO layer (100 nm) was sputtered with ZnO ceramic target by DC sputtering (conditions for sputtering, pressure = 2.7 Pa, in the presence of 80% argon and 20% oxygen) and the current required was 0.25 A DC. Acetone removed the photoresist layer leaving behind seed layer of ZnO. The devices were suspended with ZnO seed layer inverted in the solution of zinc nitrate, hexamethylenetriamine, and polyethylenimine. The bottle was incubated for 4 h at 95°C. The substrates were taken off from the solution,

The characterization was done by using various spectroscopic techniques such as X-ray diffraction, scanning electron microscopy, and transmission electron micros-

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].

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

Transmission electron microscopy (Philips FEI Tecnai 20ST) at an accelerating

/min. Field emission

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

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.

voltage of 200 kV investigates the detailed structure of ZnO NRs [29].

conducted from 20 to 60<sup>o</sup> in the 2θ range with scanning rate 5o

#### 2.5 RF sputtering

A seeding of ZnO was sputtered (5 nm) on the recommended area of CMOS devices and illustrated with metal shadow mask. The as-prepared substrate was dipped in 25 mM equimolar solution of zinc nitrate hexahydrate (Zn(NO3)26H2O) and hexamethylenetetramine and was maintained for 2 h at 90°C. The device was removed from the nutrient solution after ZnO NWs' growth washed with deionized water and dried. This process is suitable for simultaneously growing ZnO NWs on microhotplate and provides an economic process for thin film level growth as well [60].

By RF sputtering process, deposit a layer of 50 nm on the glass substrate. Wash the substrate with standard process and keep the substrate in its holder. The NRs of ZnO:Ti were grown onto the ZnO sputtered by reactive evaporation with hot-wire resistance in a furnace. After the growth of ZnO:Ti NRs, the Pt electrodes were deposited by photolithography and liftoff technique on the substrate. For better ohmic contact, anneal the samples in an Ar ambient atmosphere for 15 min at 350°C [61].

Prepare nanostructured thin film with pure ZnO (25 at. wt%) and MoO3 for gas response. Deposit it on the surface of as-prepared alumina sensor chips by sputtering and annealing process. The sensor chips containing Au-IDE and thin film Pt-heater structures were comprehended by means of DC magnetron sputtering, plasma etching, and UV-lithography. Gold wires (diameter of 50 μm) were made to contact for sensor testing body that mounted on TO-8 header by microwelding method [62].

#### 2.6 Carbothermal reduction

In a particular growth mechanism, 0.3 g zinc powder and 0.6 g effective carbon were adequately mixed in an agate mortar. The mixture was transferred into an alumina boat and kept in a horizontal muffle furnace at 900°C with 25°C/min for 2 h under normal pressure. Finally, the yellowish product formed and massed in the boat [63].

Carbothermal reduction technique can be used to synthesize ZnO NWs from graphite and ZnO powder in horizontal muffle furnace. At the downstream, the Si wafer sputtered with a 30<sup>o</sup> A Au coating. A rotary pump was used to evacuate the quartz tube at approximately 10<sup>2</sup> Torr, and Ar gas was introduced at 100 sccm. The temperature was increased to 900°C, and O2 gas was introduced with 2 sccm for 30 min. After the growth of single crystalline ZnO NWs, an SnO2 shell layer was deposited, and the temperature of the furnace cooled from 900 to 700°C. Tetramethyltin ((CH3)4Sn, 99.999%, UP Chemical Co., Ltd.) was kept in the quartz tube at 0.5 sccm of Ar as carrier gas and introduced with 2 sccm oxygen for 10 min [64].

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