2.7 Catalyst effect on the growth of metal oxide semiconductors

Thickness of the catalyst layer coated on the substrate plays a vital role in the growth of MOS nanostructure materials by reducing the activation energy of the reaction without taking part in the chemical reaction.

In supersaturation state catalyst droplet acts as a sink for source material in vapor-liquid-solid mechanism. The supersaturation level of droplet becomes smaller than the surrounding atmosphere's supersaturation level, when supersaturation of catalyst occurs. This difference creates a driving force, which drives the precursor vapors into the droplet, and growth of 1-D structures takes place in energetically favored crystallographic directions.

In vapor-solid mechanism, various types of substances are used as catalyst for the growth of 1-D nanostructures. The size and morphology of nanostructures can be controlled by using various types and thicknesses of catalysts. The finest catalyst has ideal rough surface whose sticking coefficient for the impinging of precursor material's atom from vapor phase is almost 1 [39].

### 2.8 Effect of gold catalyst on growth

Owing to its high surface tension, high accommodation coefficient, and high sticking power, gold (Au) is generally used as a catalyst in the synthesis of 1-D oxide nanostructure. Growth of 1-D oxide nanostructures with high crystallinity, density, Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors DOI: http://dx.doi.org/10.5772/intechopen.86815

and long controlled diameter can be obtained by using Au as a catalyst. Growth of 1-D nanostructures has been reported by Borchers et al. with high density using Au catalyst [40]. ZnO nanowires can be grown through VLS mechanism by adding the catalyst substance which provides the nucleation sites for the growth of nanowires. The formation of these nuclei takes place through internal chemical reaction. This is considered to be a self-catalytic VLS growth. During the growth process, the reaction at low temperature can be fastening for vapor generation by adding some external materials in the source material. ZnO powder has a melting point of 1975° C, so pure ZnO does not sublimate at 900–1100°C. So for this purpose carbon powder is mixed with ZnO power with equal mass ratio that gives rise to the formation of Zn or Zn suboxide vapors at 1000°C [41], i.e.,

$$\text{ZnO} + \text{C} \to \text{Zn} + \text{CO} \tag{4}$$

$$\text{ZnO} + \text{CO} \rightarrow \text{Zn} + \text{CO}\_2 \tag{5}$$

$$\text{ZnO} + (\text{1} - \text{x})\text{CO} \rightarrow \text{ZnO}\_x + (\text{1} - \text{x})\text{CO}\_2 \tag{6}$$

Various forms of ZnO nanostructures grow even at lower temperature because Zn or Zn suboxides act as nucleation sites for ZnO nanostructures. Other parameters like vacuum conditions, carrier gases, and catalysts are not essential in this condition. So the temperature is the only parameter that plays a vital role in the formation of various kinds of ZnO nanostructures. The formation of CO takes place by the direct reaction between graphite (C) and ZnO or O2 depending upon the reaction condition (tube condition).

The formations of suboxides take place in open quartz tube due to the partially oxidized Zn vapor or droplet by the addition of graphite (C) at low melting temperature. Due to the high reactive power of suboxides as compared to ZnO, the deposition of zinc at the tips of grown nanostructures may increase during the synthesis process [42]. It is the main advantage of self-catalytic growth that impurity-free growth can be obtained as compared to catalyst-assisted growth of VLS.

#### 2.9 Temperature effect on the growth of 1-D nanostructures

Temperature plays a crucial role in the growth of 1-D oxide nanostructures by thermal evaporation method through vapor-liquid-solid mechanism.

The thermodynamic phenomena like stability, dissociation adsorption, surface diffusion, and solubility of certain phases can be directly affected by temperature.

There are three types of ZnO fast growth direction from the structure point of view, namely, <2�1�10>, <01�10>, and � [0001], as shown in Figure 5. ZnO consists of various structures due to the polar surface activities of different growth facets. Every crystal has a unique crystal plane with different kinetic parameters, which are to be considered under controlled growth conditions.

The tetrahedral coordination of ZnO is shown, which has noncentral symmetry and piezoelectric effect [43, 44]. [0001] is the fastest growing direction which is along the c-axis because its activation temperature is lower than other two directions. Due to activation, energy growth of nanorods with smaller lengths and diameters takes place at lower temperature, but when temperature increases, length and diameter of nanowires increase because the energy of this fast-growing direction [0001] increases. At the higher temperatures, nanobelts with further increase in temperature facets <2�1�10> and <0�1�10> get high activation energies to grow nanosheets.

The supersaturation of liquid droplet (that acts as nucleation's site) with the source material vapors results in crystal structures of source material at the liquidsolid interface on the substrate, consequently forming one-dimensional nanostruc-

Schematic illustration of VLS mechanism for ZnO nanorod catalyst droplets at the tip of nanorods.

Despite the growth of 1-D oxide semiconductor nanostructures such as ZnO, GaN, and nanowires, the vapor transport process is the most dominant and costeffective synthesis method; other growth methods such as electrochemical deposition (ECD), sol-gel, polymer assisted growth, etc. have been developed so far in parallel [39]. The possibility of forming ZnO nanostructures even at low tempera-

Thickness of the catalyst layer coated on the substrate plays a vital role in the growth of MOS nanostructure materials by reducing the activation energy of the

In supersaturation state catalyst droplet acts as a sink for source material in vapor-liquid-solid mechanism. The supersaturation level of droplet becomes smaller than the surrounding atmosphere's supersaturation level, when supersaturation of catalyst occurs. This difference creates a driving force, which drives the precursor vapors into the droplet, and growth of 1-D structures takes place in

In vapor-solid mechanism, various types of substances are used as catalyst for the growth of 1-D nanostructures. The size and morphology of nanostructures can be controlled by using various types and thicknesses of catalysts. The finest catalyst has ideal rough surface whose sticking coefficient for the impinging of precursor

Owing to its high surface tension, high accommodation coefficient, and high sticking power, gold (Au) is generally used as a catalyst in the synthesis of 1-D oxide nanostructure. Growth of 1-D oxide nanostructures with high crystallinity, density,

2.7 Catalyst effect on the growth of metal oxide semiconductors

reaction without taking part in the chemical reaction.

energetically favored crystallographic directions.

material's atom from vapor phase is almost 1 [39].

2.8 Effect of gold catalyst on growth

108

ture as shown in Figure 4.

Figure 4.

Gas Sensors

2.6.2 Other synthesis methods

ture may be provided by these methods.

3.1 Preparation of substrate for growth

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

were used through the following steps:

30 min at room temperature.

growth of ZnO nanostructures.

of ZnO nanostructures.

mechanism

111

glass substrates for preparation of sensor.

By using the diamond cutter, Si substrates were cut in suitable sizes and shapes. In order to avoid the contamination, the substrates were cleaned before the deposition of catalyst, as oily layer and dust particles may stick to the surface of the substrates. For the cleaning purpose, the acetone was poured into a beaker, and the beaker was filled up to half level. The substrates were put into the acetone-filled beaker to completely immerse in them. The acetone-filled beaker was placed in ultrasonic bath at room temperature for 30 min. Si (100) substrates were then put

into ethanol and deionized water for decontamination purpose for 30 min.

For the growth of 1-D ZnO nanostructures, n-type silicon substrates Si (100)

1.Si substrates were cleaned in isopropyl alcohol (IPA), acetone, and deionized water (DI) by sonication to remove the contaminations in ultrasonic bath for

3. In nm, a thin layer of gold catalyst was deposited on Si (100) substrates for the

4.Around 200 nm of thin layer of gold catalyst was deposited on Si (100) and

5.The samples were taken out from UHV chamber and used for growth process

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

3.3 Preparation of samples by vapor transport method through VLS

2. Sample substrates were loaded in the ultrahigh vacuum chamber for deposition of thin film of gold under vacuum of 10<sup>7</sup> Torr.

3.2 Deposition of Au catalyst on Si substrate in UHV chamber

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

Figure 5. (a) Wurtzite structure. (b) Growth direction model of ZnO.
