3.3 Preparation of samples by vapor transport method through VLS mechanism

The growth was performed by thermal evaporation in a temperature-controlled horizontal tube furnace by vapor transport process through VLS mechanism. An equimolar mixture (mixed in a ball mill for 2 h with 250 rpm) of ZnO (purity 99.99%) and graphite (purity 99.9%) was placed in a ceramic boat (88 mm of length) with a mass ratio1:1 (measured by physical balance). This boat containing the source material (mixture of ZnO + C) was placed at the center of quartz tube (length 100 cm and diameter 3.5 cm). Tube furnace was set at a temperature of 850, 900, 950, and 1030°C for the four different experiments. Catalyst-coated substrates of 4 nm labeled as S1, S2, S3, and S4 were placed at the downstream of the source material at a distance of 18 cm (S1, 850°C), 12 cm (S2, 900°C), 9 cm (S3, 950°C), and 6 cm (S4, 1030°C), respectively. Furnace temperature was raised at the rate of 10°C per minute. At the start Ar gas (99.99%) was introduced at a rate of 50 standard cubic centimeter per minute (sccm) to flush out the residual present in the tube. Brass rod fitted in the rubber cork was inserted in the quartz tube to connect it to argon (Ar) gas source through a plastic pipe of 5 mm diameter. Argon gas was used as a carrier for transport of vapors from source material to gold-coated substrates. The other end of quartz tube was kept opened. The temperature of the furnace was increased from room temperature to 850°C (S1), 900°C (S2), 950°C

2.10 Doping of nanostructures

(a) Wurtzite structure. (b) Growth direction model of ZnO.

Figure 5.

Gas Sensors

3. Experimental procedure

VLS mechanism

110

Doping of the nanowires and nanorods through in situ or post processing techniques will provide a far more favorable approach to modulate their electrical, optical, and piezoelectric properties. Most metal dopant ions result in the increase of density of the conduction carriers by occupying the lattice sites in the ZnO crystal. The complete picture of the crystal can be changed by changing the doping level. The controlled modification of morphological features as well as enhancement of electrical and optical properties can be achieved by introducing dopant element in metal oxide semiconductor [45]. The electrical as well as optical properties of MOS can be tuned by adding the foreign elements or by the alternation of oxygen stoichiometry. By making these changes, one can get an increase in carrier's concentration, electrical resistivity, and mobility [45]. Doped nanostructure-based sensors are fully capable of sensing different harmful gases, with good stability, selectivity, and sensitivity. Out of many other methods, doping is considered to be

one of the best methods for enhancement of gas sensing properties of ZnO nanostructures at room temperature. Doped ZnO nanostructures were used in the past by many researchers for the detection of harmful gases in the environment. For example, the gas sensing properties of Sn-doped ZnO nanostructures were investigated by S.C. Navel and I.S. Mulla using the thermal evaporation method. The results show good response to different gases for pure Sn-doped nanostructures, in temperature range of 275°C to 300°C. They proved that the sensitivity toward UV

2.Coating of Au catalyst in ultrahigh vacuum (UHV) chamber on Si substrate

3.Preparation of nanostructure samples by vapor transport method through

sensing can be increased by the doping of Sn material [46].

Experimental process comprises the following steps:

4.Fabrication of sensor for toxic gas sensing applications

1.Preparation of substrate for growth

(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.

furnace at 400°C for 2 h. The annealing process was usually done for attachment of oxygen on the surface of ZnO nanostructures. The nanostructures were scratched with the help of blades, and the gaps or cuts on gold-coated quartz substrate were filled with the scratched nanostructures as shown in Figure 6. A small drop of methanol was dropped on the nanostructures with the help of 5 cc disposable syringe so that a thick paste was formed. The sensor was then placed under IR (infrared) light for 10 min for the purposes of sticking material on the quartz substrate. The experimental setup for chemical sensing is shown in Figure 6.

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

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. Morphological properties of ZnO nanostructures

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

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

on n-type Si (100) substrate.

Figure 7.

113

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 50 sccm, respectively.
