**4. Methodologies for achieving 1D ZnO nanostructures**

ZnO nanostructures have already been synthesized by various methods. There are mainly two methods to prepare 1D ZnO according to the state-of-growth medium: vapor phase process and liquid phase process. The vapor phase method includes chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), etc. The liquid phase method includes hydrothermal method, electrochemical method, sol-gel method, and so on. The growth mechanism can be classified into mainly three categories: vapor-solid (VS) growth, vapor-liquid-solid (VLS) growth, and solution-liquid-solid (SLS) growth.

### **4.1 Chemical vapor transport and condensation (CVTC)**

The chemical vapor transport and condensation (CVTC) method is processed in a tube furnace. The substrates coat with a layer of Au thin film with thermal evaporation methods to control the thickness of the film in the CVTC system. ZnO powder and graphite powder in the same amounts are mixed together and milled effectively; then put them into a boat made by alumina. Then, the alumina boat and the substrate with Au film are placed into a small quartz tube. Usually, the substrates are kept 5–10 cm away from the alumina boat center. At the furnace center, the boat of the alumina sits, and at the downstream of the argon flow, the substrates are positioned. The system is raised to 800–900°C and maintained for 5–30 min. Light or dark gray materials are attained on the surface of the substrate.

The transmission electron microscope (TEM) image of an alloy tip on a thin nanowire is shown in **Figure 2**. According to the features of the alloy tip, it indicates a clear growth procedure by vapor-liquid-solid (VLS) growth mechanism. The synthesized tips of nanowires shown in **Figure 2a** are grown by both the hydrogen and graphite reduction approaches. The relevant high-resolution transmission electron microscope (HRTEM) image of a single-crystalline ZnO nanowire is shown in **Figure 2b**. It expresses that the space of the adjacent lattice planes of 2.56 ± 0.05 Å is consistent to the distance of two crystal planes in (002). Consequently, it indicates that the growth direction for the ZnO nanowires is <001>. Meanwhile, it is also observed in the high diffraction intensity of (001) peaks of XRD results to confirm that the preferential growth direction is <001> [49, 50].

**Figure 3** expresses a representative image of a patterned ZnO nanowire synthesized on silicon (Si) substrate with the patterned Au islands taken by scanning electron microscopy (SEM). The diameters of the nanowires are from 20 to 120 nm,

**57**

**Figure 3.**

*magnification SEM image for the same sample.*

*Methodologies for Achieving 1D ZnO Nanostructures Potential for Solar Cells*

and the length is 5–20 μm. The directions of the grown nanowires on (100) Si

*(a) A thin ZnO nanowire image taken by TEM with a Zn/Au alloy tip. (b) the lattice fringes of high-*

SEM results show that the flexible, long, fine ZnO nanowires grow extensively from the hexagons' edges. The growth of nanowires is consistent with the copper grid in the hexagonal pattern abundantly. Interestingly, a complicated intricate network is formed due to lots of the nanowires connecting with the neighboring metal hexagons. SEM images for the higher magnification of the same sample illustrate

ZnO NWs grown under the vapor-liquid-solid process begin together with the reductive Zn gaseous reactants dissolution into the Au catalyst liquid droplets in nanosize, and then the alloy metal is formed followed by the supersaturation of Zn along with the single-crystalline wire growth. The schematic growth mechanism is expressed in **Figure 4**. In this method, the diameter, density, and location of ZnO NWs can be controlled according to the desired characteristics. As a result, ZnO NWs with the required properties can be attained and tailed successfully. However, the metal catalyst affects the purity of the product which can lower the performance of the nanowires [51–56].

*(a) From the patterned Au islands images of ZnO nanowire networks synthesized taken by SEM. (b) Higher* 

*DOI: http://dx.doi.org/10.5772/intechopen.83618*

substrate are random.

**Figure 2.**

more detailed particulars as shown in **Figure 3b**.

*resolution TEM image of a single-crystalline ZnO nanowire.*

*Methodologies for Achieving 1D ZnO Nanostructures Potential for Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83618*

### **Figure 2.**

*Renewable and Sustainable Composites*

ZnO nanostructure crystal growth [48].

growth, and solution-liquid-solid (SLS) growth.

**4.1 Chemical vapor transport and condensation (CVTC)**

**4. Methodologies for achieving 1D ZnO nanostructures**

of synthesized ZnO nanorods is about 4 μm with the diameter of around 700 nm and ZnO nanorods with the flat top surface, and they stack one by one through polar surfaces. From the crystal structure of the ZnO, the ions of Zn and O are arranged alternatively through *c* axis where the bottom surface is O2<sup>−</sup> terminated (000-1) and the top surface is Zn2+ terminated (0001). The surfaces of the flat top explored in the nanorods of ZnO are contributed to the polar surface disappearance. In the basis solution with the weak volume, the precipitate of Zn(OH)2 solid exists in the reactant solution. Owing to the dipole interaction, Zn(OH)2 solid is taken as the polar surface that could easily make the positive and negative surfaces of ZnO crystal incorporate efficiently. Therefore, the surface energy of the polar surfaces is relatively high than that of the nonpolar surfaces, disappears at the first when the nonpolar surfaces start to slowly grow, and appears in the last stage of

ZnO nanostructures have already been synthesized by various methods. There

The chemical vapor transport and condensation (CVTC) method is processed in a tube furnace. The substrates coat with a layer of Au thin film with thermal evaporation methods to control the thickness of the film in the CVTC system. ZnO powder and graphite powder in the same amounts are mixed together and milled effectively; then put them into a boat made by alumina. Then, the alumina boat and the substrate with Au film are placed into a small quartz tube. Usually, the substrates are kept 5–10 cm away from the alumina boat center. At the furnace center, the boat of the alumina sits, and at the downstream of the argon flow, the substrates are positioned. The system is raised to 800–900°C and maintained for 5–30 min.

Light or dark gray materials are attained on the surface of the substrate.

that the preferential growth direction is <001> [49, 50].

The transmission electron microscope (TEM) image of an alloy tip on a thin nanowire is shown in **Figure 2**. According to the features of the alloy tip, it indicates a clear growth procedure by vapor-liquid-solid (VLS) growth mechanism. The synthesized tips of nanowires shown in **Figure 2a** are grown by both the hydrogen and graphite reduction approaches. The relevant high-resolution transmission electron microscope (HRTEM) image of a single-crystalline ZnO nanowire is shown in **Figure 2b**. It expresses that the space of the adjacent lattice planes of 2.56 ± 0.05 Å is consistent to the distance of two crystal planes in (002). Consequently, it indicates that the growth direction for the ZnO nanowires is <001>. Meanwhile, it is also observed in the high diffraction intensity of (001) peaks of XRD results to confirm

**Figure 3** expresses a representative image of a patterned ZnO nanowire synthesized on silicon (Si) substrate with the patterned Au islands taken by scanning electron microscopy (SEM). The diameters of the nanowires are from 20 to 120 nm,

are mainly two methods to prepare 1D ZnO according to the state-of-growth medium: vapor phase process and liquid phase process. The vapor phase method includes chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), etc. The liquid phase method includes hydrothermal method, electrochemical method, sol-gel method, and so on. The growth mechanism can be classified into mainly three categories: vapor-solid (VS) growth, vapor-liquid-solid (VLS)

**56**

*(a) A thin ZnO nanowire image taken by TEM with a Zn/Au alloy tip. (b) the lattice fringes of highresolution TEM image of a single-crystalline ZnO nanowire.*

and the length is 5–20 μm. The directions of the grown nanowires on (100) Si substrate are random.

SEM results show that the flexible, long, fine ZnO nanowires grow extensively from the hexagons' edges. The growth of nanowires is consistent with the copper grid in the hexagonal pattern abundantly. Interestingly, a complicated intricate network is formed due to lots of the nanowires connecting with the neighboring metal hexagons. SEM images for the higher magnification of the same sample illustrate more detailed particulars as shown in **Figure 3b**.

ZnO NWs grown under the vapor-liquid-solid process begin together with the reductive Zn gaseous reactants dissolution into the Au catalyst liquid droplets in nanosize, and then the alloy metal is formed followed by the supersaturation of Zn along with the single-crystalline wire growth. The schematic growth mechanism is expressed in **Figure 4**. In this method, the diameter, density, and location of ZnO NWs can be controlled according to the desired characteristics. As a result, ZnO NWs with the required properties can be attained and tailed successfully. However, the metal catalyst affects the purity of the product which can lower the performance of the nanowires [51–56].

### **Figure 3.**

*(a) From the patterned Au islands images of ZnO nanowire networks synthesized taken by SEM. (b) Higher magnification SEM image for the same sample.*

**Figure 4.** *Schematic mechanism of ZnO NW growth.*
