**4.2 Chemical vapor deposition (CVD)**

The aligned 1D ZnO NWs fabricated by patterning metal catalytic particles through VLS growth involve tedious lift-off processes for patterning metal catalysts and may lead to serious contamination in complementary metal oxide semiconductor processing. To avoid the effects of the catalyst, CVD without the catalyst has been developed. The zinc oxide NW growth is proceeded and developed earlier when the flow-type reactor is at a reduced pressure of the elemental vapor phase synthesis. Highly purified metallic granulated Zn (99.99%) is placed in the boat made by the alumina. After that, it is inserted at a quartz ampoule end which is sealed at one end. Meantime, a wide slit in the ampoule is at the open end. The substrates are mounted with their front sides up and below the ampoule. Then, a horizontal two-zone flowtype quartz reactor is used for putting the ampoule inside in order to make sure that the source of zinc is situated in one of the zones (evaporation zone), the substrates, and in the other (growth zone). The arrays of ZnO growth are very allergic to the processing parameters, especially for the well-aligned nanorods. Wang ZL and his co-workers firstly fabricated a ZnO nanobelt in 2001 using catalyst-free CVD methods and applied it to the devices [57]. Furthermore, Wang's group also achieved the formation process of a rectangular cross section by simply evaporating ZnO powder at elevated temperatures. The structures of as-synthesized nanobelts are uniform. Most of the nanobelts are single crystals and free from defects and dislocations. To obtain the ZnO nanowire arrays (NWAs) via catalyst-free CVD methods, many groups have adopted different methods to reach the objective [58–62]. One method is to adopt a ZnO film layer to induce the growth of ZnO NWs. The nucleation stage of ZnO nanocrystals plays an important role. Firstly, the zinc metal droplet condenses on the surface of the substrate due to the different heating temperatures in the evaporation and growth zones. The diameter of catalyst droplets can be controlled by zinc and oxygen partial pressure which is different from the Au catalyst growth mechanism. In addition, Menzel A et al. deeply investigated the method for tuning the growth mechanism of ZnO nanowires under the various conditions and put out the related parameters for a controlled NW growth by CVD method to change the nanowire shapes. The results are shown in **Figure 5** [63].

### **4.3 Metal-Organic chemical vapor deposition (MOCVD)**

The MOCVD method, apart from its increasing advantages because of its unique characters of the industry, has been illustrated to be effectively taken to ZnO NW synthesis with good controllable shape, high quality, and reproducibility. The growth of ZnO NWs with high quality by catalyst-free MOCVD was explored firstly by Park et al. [64]. After that, the huge effort has been put into the field of

**59**

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

the synthesis of the ZnO nanostructures, and great progress has subsequently been obtained [65–69]. Up to now, the contributions relevant to MOCVD growth of ZnO nanostructures, diethylzinc (DEZn), dimethylzinc (DMZn) as zinc precursor, nitrogen or argon as carrier gas, and low reactor-pressures have been mostly used. Park's group reported the growth of ZnO NWs, which requires no metal catalysts. The nanorod growth temperature was as low as 400°C. However, the preparation machine and source materials are more expensive than other methods which have

Chemical solution deposition is one of the commonly employed synthesis methods for ZnO nanostructures, particularly in large-scale fabrication for device purposes [70–75]. Chemical reactions between different precursors play a key role in the synthesis. The advantages of the solution method include the economical synthesis, the large scale, and the low temperature. For ZnO nanomaterials growing on a substrate, the approaches of solution mostly adopt the hydrothermal procedure by a kind of solution in an aqueous which includes an organic amine and zinc salt. In addition, in order to enhance the ZnO NR alignment on the substrate, a textured

In a typical chemical solution synthesis, a layer of ZnO seed layer is spread over a Si substrate by dipping or sputtering. This kind of seeding method is simply suitable for different substrates. The ZnO seed layer thickness is usually 10–200 nm. A calculated amount of zinc nitrate hexahydrate is dissolved in 80 mL deionized water to obtain 1–40 mM solutions for growth solution preparation. Then, the pH of the solution for ZnO growth is adjusted by ammonia water addition. The amount of the ammonia water addition related to the target pH and the zinc salt concentration is usually about 0.1–5 mL. The growth of the hydrothermal ZnO NRs is explored

ZnO nanocrystal or a ZnO thin film is taken as a seed layer.

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

hindered its practical applications.

*Schematic diagram of the key parameters controlling the NW growth.*

**4.4 Chemical solution method**

**Figure 5.**

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

**Figure 5.**

*Renewable and Sustainable Composites*

**4.2 Chemical vapor deposition (CVD)**

*Schematic mechanism of ZnO NW growth.*

**Figure 4.**

nanowire shapes. The results are shown in **Figure 5** [63].

**4.3 Metal-Organic chemical vapor deposition (MOCVD)**

The MOCVD method, apart from its increasing advantages because of its unique characters of the industry, has been illustrated to be effectively taken to ZnO NW synthesis with good controllable shape, high quality, and reproducibility. The growth of ZnO NWs with high quality by catalyst-free MOCVD was explored firstly by Park et al. [64]. After that, the huge effort has been put into the field of

The aligned 1D ZnO NWs fabricated by patterning metal catalytic particles through VLS growth involve tedious lift-off processes for patterning metal catalysts and may lead to serious contamination in complementary metal oxide semiconductor processing. To avoid the effects of the catalyst, CVD without the catalyst has been developed. The zinc oxide NW growth is proceeded and developed earlier when the flow-type reactor is at a reduced pressure of the elemental vapor phase synthesis. Highly purified metallic granulated Zn (99.99%) is placed in the boat made by the alumina. After that, it is inserted at a quartz ampoule end which is sealed at one end. Meantime, a wide slit in the ampoule is at the open end. The substrates are mounted with their front sides up and below the ampoule. Then, a horizontal two-zone flowtype quartz reactor is used for putting the ampoule inside in order to make sure that the source of zinc is situated in one of the zones (evaporation zone), the substrates, and in the other (growth zone). The arrays of ZnO growth are very allergic to the processing parameters, especially for the well-aligned nanorods. Wang ZL and his co-workers firstly fabricated a ZnO nanobelt in 2001 using catalyst-free CVD methods and applied it to the devices [57]. Furthermore, Wang's group also achieved the formation process of a rectangular cross section by simply evaporating ZnO powder at elevated temperatures. The structures of as-synthesized nanobelts are uniform. Most of the nanobelts are single crystals and free from defects and dislocations. To obtain the ZnO nanowire arrays (NWAs) via catalyst-free CVD methods, many groups have adopted different methods to reach the objective [58–62]. One method is to adopt a ZnO film layer to induce the growth of ZnO NWs. The nucleation stage of ZnO nanocrystals plays an important role. Firstly, the zinc metal droplet condenses on the surface of the substrate due to the different heating temperatures in the evaporation and growth zones. The diameter of catalyst droplets can be controlled by zinc and oxygen partial pressure which is different from the Au catalyst growth mechanism. In addition, Menzel A et al. deeply investigated the method for tuning the growth mechanism of ZnO nanowires under the various conditions and put out the related parameters for a controlled NW growth by CVD method to change the

**58**

*Schematic diagram of the key parameters controlling the NW growth.*

the synthesis of the ZnO nanostructures, and great progress has subsequently been obtained [65–69]. Up to now, the contributions relevant to MOCVD growth of ZnO nanostructures, diethylzinc (DEZn), dimethylzinc (DMZn) as zinc precursor, nitrogen or argon as carrier gas, and low reactor-pressures have been mostly used. Park's group reported the growth of ZnO NWs, which requires no metal catalysts. The nanorod growth temperature was as low as 400°C. However, the preparation machine and source materials are more expensive than other methods which have hindered its practical applications.

### **4.4 Chemical solution method**

Chemical solution deposition is one of the commonly employed synthesis methods for ZnO nanostructures, particularly in large-scale fabrication for device purposes [70–75]. Chemical reactions between different precursors play a key role in the synthesis. The advantages of the solution method include the economical synthesis, the large scale, and the low temperature. For ZnO nanomaterials growing on a substrate, the approaches of solution mostly adopt the hydrothermal procedure by a kind of solution in an aqueous which includes an organic amine and zinc salt. In addition, in order to enhance the ZnO NR alignment on the substrate, a textured ZnO nanocrystal or a ZnO thin film is taken as a seed layer.

In a typical chemical solution synthesis, a layer of ZnO seed layer is spread over a Si substrate by dipping or sputtering. This kind of seeding method is simply suitable for different substrates. The ZnO seed layer thickness is usually 10–200 nm. A calculated amount of zinc nitrate hexahydrate is dissolved in 80 mL deionized water to obtain 1–40 mM solutions for growth solution preparation. Then, the pH of the solution for ZnO growth is adjusted by ammonia water addition. The amount of the ammonia water addition related to the target pH and the zinc salt concentration is usually about 0.1–5 mL. The growth of the hydrothermal ZnO NRs is explored

### *Renewable and Sustainable Composites*

with Zn/Si substrate suspended upside down in a kind of Teflon-capped glass bottle which is full of the growth solution. The temperature of ZnO growth ranges from 60 to 90°C and the reacting time is 6 h. When the synthesis is finished, the substrate is taken from the reactant solution. At the same time, the substrate is rinsed by the DI water and dried successively. Therefore, the morphology (length, diameter) of the synthesized nanorods relied on the relevant parameters, for instance, zinc seed layer morphology, pH, growing temperature, and zinc salt concentration.

**Figure 6a** shows ZnO nanorods viewed normal to the surface grown by the two-step chemical bath deposition process, in which the facets are exactly crystalline hexagonal and its average ratio of the aspect is about 3 ± 1 [76]. **Figure 6c** illustrates the sulfidation of ZnO nanorods with definitely well particle decoration of the entire surface of ZnO nanorods, and the end and side of facets are not in the well-defined morphology. With the reaction increasing, a more uniform film coated

### **Figure 6.**

*ZnO nanorod (a) initial morphology (sample A) before sulfidation, (c) sulfidation for 90 min at 75°C in 160 mmol Na2S(aq) (sample B), and (e) treatment further for 90 min at 75°C in 160 mmol Zn(NO3)2·6H2O(aq) (sample C). EDS image (b) results of a, (d) results of c, and (f) results of e.*

**61**

**Figure 7.**

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

on ZnO nanorods can be easily observed as shown in **Figure 6e**. Moreover, the side and end facets of the synthesized ZnO nanorods as shown in **Figure 6e** become more smooth (cf. **Figure 6a**, **Figure 6c**, and **Figure 6e**). Meanwhile, EDX results

The related TEM results are shown in **Figure 7**. It expresses that ZnO nanorod is coated with an uneven film. The selected area diffraction (SAD) result as shown in

*Sample B images of HRTEM. (a) Sulfidation nanorod (bright field), (b) region in blue ellipse (***Figure 7a***) (selected area diffraction (SAD) pattern), (c and e) interface of ZnO-ZnS (high-resolution images), (d) region in red (***Figure 7c***) live Fourier transformation, and (f) region in red (***Figure 7e***) live Fourier transformation.*

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

agree well with the relevant SEM results.

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

*Renewable and Sustainable Composites*

with Zn/Si substrate suspended upside down in a kind of Teflon-capped glass bottle which is full of the growth solution. The temperature of ZnO growth ranges from 60 to 90°C and the reacting time is 6 h. When the synthesis is finished, the substrate is taken from the reactant solution. At the same time, the substrate is rinsed by the DI water and dried successively. Therefore, the morphology (length, diameter) of the synthesized nanorods relied on the relevant parameters, for instance, zinc seed

layer morphology, pH, growing temperature, and zinc salt concentration.

**Figure 6a** shows ZnO nanorods viewed normal to the surface grown by the two-step chemical bath deposition process, in which the facets are exactly crystalline hexagonal and its average ratio of the aspect is about 3 ± 1 [76]. **Figure 6c** illustrates the sulfidation of ZnO nanorods with definitely well particle decoration of the entire surface of ZnO nanorods, and the end and side of facets are not in the well-defined morphology. With the reaction increasing, a more uniform film coated

**60**

**Figure 6.**

*ZnO nanorod (a) initial morphology (sample A) before sulfidation, (c) sulfidation for 90 min at 75°C in 160 mmol Na2S(aq) (sample B), and (e) treatment further for 90 min at 75°C in 160 mmol Zn(NO3)2·6H2O(aq)*

*(sample C). EDS image (b) results of a, (d) results of c, and (f) results of e.*

on ZnO nanorods can be easily observed as shown in **Figure 6e**. Moreover, the side and end facets of the synthesized ZnO nanorods as shown in **Figure 6e** become more smooth (cf. **Figure 6a**, **Figure 6c**, and **Figure 6e**). Meanwhile, EDX results agree well with the relevant SEM results.

The related TEM results are shown in **Figure 7**. It expresses that ZnO nanorod is coated with an uneven film. The selected area diffraction (SAD) result as shown in

### **Figure 7.**

*Sample B images of HRTEM. (a) Sulfidation nanorod (bright field), (b) region in blue ellipse (***Figure 7a***) (selected area diffraction (SAD) pattern), (c and e) interface of ZnO-ZnS (high-resolution images), (d) region in red (***Figure 7c***) live Fourier transformation, and (f) region in red (***Figure 7e***) live Fourier transformation.*

**Figure 7b** assures the crystalline structure in which the bright spots are the crystalline ZnO and the rings are the polycrystalline ZnS. The interplanar distances related to ZnO (01-12) and (10-10) and ZnS (111), together with the relationship of the partial epitaxial between ZnS shell and ZnO core where (10-10) ZnO//(111) ZnS, are confirmed. Furthermore, the calculated parameters of the lattice of ZnO core are 5.35 ± 0.01 Å at the *c*-axis and 3.29 ± 0.01 Å at the *a*-axis.
