4. Morphological properties of ZnO nanostructures

Morphology, size, and shape of the synthesized ZnO nanostructures were characterized by using scanning electron microscopy (SEM) characterization technique. The four samples were synthesized at different temperatures with the same flow rate of 50 sccm of Ar (argon) gas and with same growth time of 45 min. A total eight samples was prepared in four different experiments; out of eight samples, four samples were optimized. Four experiments were done at different temperatures, i.e., 850, 900, 950, and 1030°C. The catalyst used was 4 nm thin layer of gold coated on n-type Si (100) substrate.

Figure 7.

(S3), and 1030°C (S4) in four different experiments. When the temperature of the furnace reached the set temperature, the dwell or growth time was noted for 45 min. After 45 min the furnace program was "OFF," and the temperature started decrease gradually. When the temperature decreased to 650°C, the Ar gas flow was switched "OFF." Furnace was then cooled to room temperature after the reaction. Doping of Mg was carried out, and for that purpose 0.05 g and 0.08 g of magnesium acetate [Mg(CH3COO)24H2O] (purity 99.99%) was added in 1 g of source material (ZnO + C). Mg-doped ZnO nanostructures were synthesized by thermal evaporation in a temperature-controlled horizontal furnace on an Aucoated Si (100) substrate. Vapor transport method has been used for the synthesis of Mg-doped ZnO nanostructures which was done in a temperature-controlled tube furnace. The temperature, growth time, and gas flow rate were 900°C, 45 min, and

The synthesized ZnO nanostructures were used for UV as well as for chemical sensing applications. ZnO nanostructures were annealed by heating it in digital

> Synthesis temperature (°C)

Gas flow rate (sccm)

Growth time (min)

Catalyst thickness (nm)

S1 ZnO + C Au 4 850 50 45

50 sccm, respectively.

Gas Sensors

3.4 Sensor fabrication

Samples Source

Table 1.

material

Catalyst used

S2 900 S3 950 S4 1030

Sample details (synthesis temperature, gas flow rate, growth time).

Figure 6.

112

Schematic illustration of chemical sensing experimental setup.

SEM images of different morphologies of ZnO nanostructures at different synthesized temperatures. (a) SEM images of nanowires grown at 850°C. (b) SEM images of nanorods grown at 900°C. (c) SEM images of nanobelts with needle-like ends grown at 950°C. (d) SEM images of nanobelts grown at 1030°C.

#### 4.1 Sample S1

In the first experiment, ZnO nanowires with various dimensions were obtained. Figure 7(a) shows the SEM micrograph of the ZnO nanostructures of sample S1, consisting of randomly oriented ZnO nanowires. These nanowires were grown at a temperature of 850°C on a thin layer of pure gold-coated Si (100) substrate. The nanowires intertwine with each other and distribute on the whole substrate surface randomly. The average diameter and the average length are 0.95 0.11 μm and 35.59 9.90 μm, respectively.

### 4.2 Sample S2

In the second experiment, ZnO nanorods of different dimensions were obtained. Figure 7(b) shows the SEM micrograph of complex ZnO nanorods of sample S2. These complex nanorods were grown at temperature of 900°C on a thin layer of gold-coated Si (100) substrate. The average diameter and the average length of S2 SEM images are 12.66 3.72 μm and 319.48 93.50 μm, respectively.

#### 4.3 Sample S3

In the third experiment, ZnO nanobelts with needle-like ends were obtained. Figure 7(c) shows the SEM micrograph of ZnO nanobelts of sample S3 with needlelike ends. These nanobelts were obtained with different dimensions at temperature of 950°C grown on 4 nm Au-coated thin layer of Si substrate. The average width, average length, and average thickness of tips are 1.39 0.44 μm, 10.34 2.71 μm, and 0.38 0.086 μm, respectively.

ZnO þ C ! Zn þ CO (7)

ZnO þ CO ! Zn þ CO2 (8)

4.6 Synthesis of Mg-doped ZnO nanostructures

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

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

ture of the furnace was maintained at 850°C.

two steps:

115

Figure 8.

The whole process of Mg-doped ZnO nanowires could be explained in

(a) SEM images of ZnO nanowires and (b) EDX image of the corresponding ZnO nanowires grown at 900°C.

used to wash the tube, and last ethanol was used to clean the tube.

In the first step, a thin layer (4 nm) of Au film was coated on Si (100) substrate in UHV chamber by ion sputtering technique. Cleaning of Si (100) substrates was carried out by sonicating in acetone, ethanol, and deionized water for 30 min. Si (100) substrates were then coated with SiO2 for 2 h at temperature of 1050°C for insulation purpose. The quartz tube was cleaned first with chromosulfuric acid (cleaning agent) to remove the permanent residue, then the deionized water was

In the second step, the Mg-doped ZnO nanostructures were grown by vapor transport method through VLS mechanism in a temperature-controlled digital horizontal furnace as shown in the schematic illustration. In the first experiment, doping of sample S1 (nanowires) was carried out. A mixture of ZnO (purity 99.9%), magnesium acetate [Mg(CH3COO)2�4H2O] (purity 99.99%), and graphite powders (carbon) with mass ratio in gram (weighted by physical balance) of 1:1:0.05 was used as the source materials. The source material was placed at the center of quartz tube of length 100 cm and diameter 3.5 cm in a ceramic boat of 88 mm length. Sample S1 substrate was placed on a second ceramic boat at the downstream at a distance of 18 cm away from the source materials in the quartz tube. The tempera-

At the start Ar gas was introduced at the rate of 50 sccm to flush out the residual present in the tube. As the temperature reached 850°C, the dwell time was noted for 45 min. After 45 min the furnace program was "OFF," and the temperature started to decrease gradually. When the temperature decreased to 650°C, the Ar gas flow was switch "OFF." Furnace was then cooled down to room temperature after the reaction. In the second experiment, Mg doping of sample S2 (ZnO nanorods) was carried out. The same condition and parameters were used for doping of S2, except

#### 4.4 Sample S4

Figure 7 shows the SEM micrograph of ZnO nanobelts of the fourth experiment which was grown at 1030°C on gold-coated Si substrate. The average length of 2.67 0.42 μm, average width of 0.33 0.03 μm, and the average thickness of 0.09 0.01 μm of nanobelts were obtained.

The scanning electron micrographs clearly showed that the morphologies tuned from nanowires and nanorods to nanobelts due to change in temperature. High temperature and supersaturation conditions lead to the formation of nanobelts with needle-like ends and typical nanobelts.

The possible reason for this tune in morphologies is attributed to supersaturation, growth rate, and quick availability of ZnO polar surfaces for growth [46]. Overall, the supersaturation conditions are different at different temperatures which eventually change the morphology.

#### 4.5 Energy diffraction X-ray (EDX) analysis

EDX spectroscopy analytic technique was used for the chemical composition analysis of the synthesized ZnO nanostructures. Figure 8 shows the typical EDX spectrum of the sample S1 (ZnO nanowires). Only the Zn, O, and Au peaks were observed. The observation of Au peak may suggest that the growth is catalystassisted [47–52]. The approximate atomic ratio was found to be 58:32. These ratios show nonstoichiometry, i.e., crystal defects of grown nanostructures during the growth process. Deviation from the stoichiometry is large due to carbothermal reaction and oxygen-deficient environment (Ar gas) during the growth process. Most of the oxygen is used in the formation of CO2, i.e.,

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors DOI: http://dx.doi.org/10.5772/intechopen.86815

Figure 8. (a) SEM images of ZnO nanowires and (b) EDX image of the corresponding ZnO nanowires grown at 900°C.

$$\text{Zn}\text{O} + \text{C} \to \text{Zn} + \text{CO} \tag{7}$$

$$\text{ZnO} + \text{CO} \rightarrow \text{Zn} + \text{CO}\_2 \tag{8}$$

#### 4.6 Synthesis of Mg-doped ZnO nanostructures

The whole process of Mg-doped ZnO nanowires could be explained in two steps:

In the first step, a thin layer (4 nm) of Au film was coated on Si (100) substrate in UHV chamber by ion sputtering technique. Cleaning of Si (100) substrates was carried out by sonicating in acetone, ethanol, and deionized water for 30 min. Si (100) substrates were then coated with SiO2 for 2 h at temperature of 1050°C for insulation purpose. The quartz tube was cleaned first with chromosulfuric acid (cleaning agent) to remove the permanent residue, then the deionized water was used to wash the tube, and last ethanol was used to clean the tube.

In the second step, the Mg-doped ZnO nanostructures were grown by vapor transport method through VLS mechanism in a temperature-controlled digital horizontal furnace as shown in the schematic illustration. In the first experiment, doping of sample S1 (nanowires) was carried out. A mixture of ZnO (purity 99.9%), magnesium acetate [Mg(CH3COO)2�4H2O] (purity 99.99%), and graphite powders (carbon) with mass ratio in gram (weighted by physical balance) of 1:1:0.05 was used as the source materials. The source material was placed at the center of quartz tube of length 100 cm and diameter 3.5 cm in a ceramic boat of 88 mm length. Sample S1 substrate was placed on a second ceramic boat at the downstream at a distance of 18 cm away from the source materials in the quartz tube. The temperature of the furnace was maintained at 850°C.

At the start Ar gas was introduced at the rate of 50 sccm to flush out the residual present in the tube. As the temperature reached 850°C, the dwell time was noted for 45 min. After 45 min the furnace program was "OFF," and the temperature started to decrease gradually. When the temperature decreased to 650°C, the Ar gas flow was switch "OFF." Furnace was then cooled down to room temperature after the reaction. In the second experiment, Mg doping of sample S2 (ZnO nanorods) was carried out. The same condition and parameters were used for doping of S2, except

4.1 Sample S1

Gas Sensors

4.2 Sample S2

4.3 Sample S3

4.4 Sample S4

114

35.59 9.90 μm, respectively.

and 0.38 0.086 μm, respectively.

0.01 μm of nanobelts were obtained.

needle-like ends and typical nanobelts.

which eventually change the morphology.

4.5 Energy diffraction X-ray (EDX) analysis

Most of the oxygen is used in the formation of CO2, i.e.,

In the first experiment, ZnO nanowires with various dimensions were obtained. Figure 7(a) shows the SEM micrograph of the ZnO nanostructures of sample S1, consisting of randomly oriented ZnO nanowires. These nanowires were grown at a temperature of 850°C on a thin layer of pure gold-coated Si (100) substrate. The nanowires intertwine with each other and distribute on the whole substrate surface randomly. The average diameter and the average length are 0.95 0.11 μm and

In the second experiment, ZnO nanorods of different dimensions were obtained. Figure 7(b) shows the SEM micrograph of complex ZnO nanorods of sample S2. These complex nanorods were grown at temperature of 900°C on a thin layer of gold-coated Si (100) substrate. The average diameter and the average length of S2

In the third experiment, ZnO nanobelts with needle-like ends were obtained. Figure 7(c) shows the SEM micrograph of ZnO nanobelts of sample S3 with needlelike ends. These nanobelts were obtained with different dimensions at temperature of 950°C grown on 4 nm Au-coated thin layer of Si substrate. The average width, average length, and average thickness of tips are 1.39 0.44 μm, 10.34 2.71 μm,

Figure 7 shows the SEM micrograph of ZnO nanobelts of the fourth experiment which was grown at 1030°C on gold-coated Si substrate. The average length of 2.67 0.42 μm, average width of 0.33 0.03 μm, and the average thickness of 0.09

The scanning electron micrographs clearly showed that the morphologies tuned from nanowires and nanorods to nanobelts due to change in temperature. High temperature and supersaturation conditions lead to the formation of nanobelts with

The possible reason for this tune in morphologies is attributed to supersaturation, growth rate, and quick availability of ZnO polar surfaces for growth [46]. Overall, the supersaturation conditions are different at different temperatures

EDX spectroscopy analytic technique was used for the chemical composition analysis of the synthesized ZnO nanostructures. Figure 8 shows the typical EDX spectrum of the sample S1 (ZnO nanowires). Only the Zn, O, and Au peaks were observed. The observation of Au peak may suggest that the growth is catalystassisted [47–52]. The approximate atomic ratio was found to be 58:32. These ratios show nonstoichiometry, i.e., crystal defects of grown nanostructures during the growth process. Deviation from the stoichiometry is large due to carbothermal reaction and oxygen-deficient environment (Ar gas) during the growth process.

SEM images are 12.66 3.72 μm and 319.48 93.50 μm, respectively.

the magnesium acetate [Mg(CH3COO)24H2O] weight was 0.08 g, and the sample distance from the source material was 12 cm.

4.7 Morphology analysis

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

Mg-doped ZnO nanostructure morphology was probed by means of SEM. Figure 9(a) shows the SEM image of undoped ZnO nanorods (S2) with average diameter and length of 12.66 3.72 μm and 319.48 93.50 μm, respectively. Figure 9(b) shows SEM images of Mg-doped (0.05 g) ZnO nanobelts. The average thickness of 1.88 0.70 μm, average width of 4.7 1.04 μm, and average length of 72.03 18.84 μm of the Mg-doped ZnO nanobelts were measured. Figure 9(c) shows the SEM image of undoped typical ZnO nanowires (S1) with different dimensions, having average diameter and average length of 0.95 0.11 μm and 35.59 9.90 μm, respectively. Figure 9(d) shows the respective EDX analysis spectrum of the undoped ZnO nanowires (S1). The EDX spectra show the attachment of O (oxygen) and Zn (zinc) in the ratio O/Zn which was found to be 32:58, respectively. These composition analyses clearly showed that no impurity peak was observed, showing the purity of ZnO nanostructures. The aspect ratio of undoped and doped ZnO nanorods and nanobelts was found to be 25 and 51, respectively. Figure 9(e) shows the Mg-doped (0.08 g) ZnO nanobelts having average thickness of 0.05 0.009 μm, average width of 0.28 0.02 μm, and average length of 2.93 0.87 μm. The corresponding elemental compositions of the synthesized ZnO nanobelts were confirmed by EDX spectroscopy. Figure 9(f) shows the

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

corresponding EDX analysis of the doped ZnO nanobelts, showing the presence of oxygen, magnesium, and zinc in the ratio O/Mg/Zn which was found to be

28:0.35:72 respectively. EDX analysis confirmed that the compositions of the products are Mg-doped ZnO without impurity. The aspect ratio of undoped ZnO nanowires and Mg-doped ZnO nanobelts was found to be 37 and 38, respectively. The possible reason for the formation of thin and transparent nanobelts is due to the morphology tuning from nanorods and nanowires to nanobelts by Mg doping, because doping of definite elements plays a key role in the alteration of the dimensions of nanostructures [52–58]. Growth rates and polar surfaces can provoke the asymmetric growth. Formation of nanobelts was explained as continuous process of

Polar surfaces of wurtzite crystals of oxide semiconductors can induce asym-

The first step was the preparation of CH4 gas sensor. In the fabrication of CH4 gas sensor, a thick layer (200 nm) of Au was coated by ion sputtering technique on Si (100) wafers. A small amount of ZnO nanostructures was put on a pair of interdigitated electrodes on Si substrates having a gap of 55 μm. A small drop of methanol was dropped on the nanomaterials so that a thick paste was formed. The annealing

performing the gas sensing experiments, for the purpose of attachment of oxygen on the surface of sensors. The sensing experiment was performed at 200°C with 5-min cycles of dry air and 400 ppm CH4 gas concentration. The sensing response (S = Ra/ Rg) of the device was measured by resistance change upon exposure to air (Ra) and CH4 gas (Rg). Figure 10(a) and (b) shows the sensitivity response of CH4 (methane gas) at 200°C for undoped ZnO nanowires and for Mg-doped ZnO nanobelts, respectively. Research papers showed that the sensitivity of the resistive sensors is highly affected by the Mg doping. The sensors were tested in a temperature range of 50–200°C for 400 ppm of CH4 gas. Sensors show some response magnitude from 100°C temperature. Undoped ZnO nanowire sensors get its optimal point at 200°C

metric growth which leads to the diverse nanostructures, e.g., nanocombs,

of sensors was carried out in an open furnace tube for 2 h at 400°C before

1-D branching and subsequent 2-D interspace filling.

nanobrushes, needle-like belts/rods, etc. [59].

4.8 CH4 gas sensing response

117

The collected Mg-doped ZnO nanostructure sample characterization was carried out for crystallinity, morphology and elemental composition, and optical properties. Optical and gas sensing response of the respective Mg-doped ZnO nanostructures was carried out by measuring respective resistances by two probe methods using a multimeter (Keithly 2100).

#### Figure 9.

(a) SEM image of undoped ZnO nanorods (S2). (b) SEM images of Mg-doped ZnO nanobelts. (c) SEM image of undoped ZnO nanowires (S1). (d) SEM images of Mg-doped ZnO nanobelts. (e) and (f) show EDX analysis of undoped and Mg-doped ZnO nanowires and nanobelts, respectively.
