**2. Crystal growth mechanisms and kinetics**

#### **2.1 Oriented attachment (OA) mechanism**

The nonclassical mechanism, named oriented attachment (OA) is the most common crystal growth mechanism to understand the aggregation-based crystal growth of materials at the nanoscale. It is the self-assembly of adjacent nanocrystals to form a secondary crystal through Brownian motion by sharing a common crystallographic orientation (**Figure 3(a)**) [54, 55]. This OA-based crystal growth was first described in 1998 for hydrothermally synthesized TiO2 nanocrystals. Penn and coworkers observed the anisotropic chain of TiO2 anatase crystals attachment across the {112} facets using a high-resolution transmission electron microscope (HR-TEM) [54, 56–58]. The OA-based crystal growth is governed by thermodynamics as the result of the reduction of total crystal surface energy [54, 59]. Highly ordered monocrystalline materials can be formed through OA, which is a versatile approach for the preparation of anisotropic 1D nanowires and nanorods.

*Ultrathin Metal Hydroxide/Oxide Nanowires: Crystal Growth, Self-Assembly, and Fabrication… DOI: http://dx.doi.org/10.5772/intechopen.101117*

**Figure 3.** *Schematic representation of (a) OA-based crystal growth and (b) grain-rotation-induced grain coalescence mechanism.*

As described by Moldovan et al., the primary nanocrystal colloids in a solution rotate for a crystal match to achieve a perfect coherent grain–grain interface in nearby crystals and start coalescence of nanocrystals, eliminating the common grain boundaries to form a single nanocrystal, shown in **Figure 3(b)** [60, 61]. This model is named as grain-rotation-induced grain coalescence (GRIGC) mechanism to describe the crystal growth process of OA-based nanomaterials [62]. This mechanism is based on the reduction of the crystal surface energy by minimizing the area of high-energy surfaces. Leite et al. have observed the OA mechanism experimentally in the growth process of SnO2 nanocrystals at room temperature [63]. With the recent advancement of liquid-phase high-resolution transmission electron microscopy (liquid phase HR-TEM), Li and coworkers directly observed the OA mechanism of iron oxyhydroxide nanoparticles [64].

The ultrathin nanowires produced by the OA process provide unique features such as constant nanowire diameter during the growth by direct attachment of nanocrystals to the tip of the growing nanowire similar to polymerization reactions [65]. Therefore, the diameter of the nanowire can be predetermined by the diameter of the nanocrystals, which are monodispersed. However, the disadvantages of OA-based nanowire synthesis methods are poor yield and having residues of ligands and solvents attached to the nanowire [11].

#### **2.2 Crystal growth kinetic models and prior arts**

The crystal growth kinetics is mainly depending on the nature of the material, interface of crystal facets, working temperature, the type of surrounding solution and the concentration of the solution [66]. In a colloidal solution, the crystal growth mechanism for the formation of metal and metal hydroxide/ oxide microstructures is often explained by Ostwald ripening (OR) theory. The OR crystal growth is controlled by diffusion, where larger particles grow at the expense of smaller particles [56, 57, 67]. The kinetic model of the OR mechanism is LSW kinetic model, which is attributed to the first-order chemical reactions, as in Eq. (1) [68, 69].

$$D^u - D\_0^u = k(t - t\_0) \tag{1}$$

where *D* and *D*0 are the mean particle sizes at time *t* and *t*0, *k* is a temperaturedependent rate constant, *n* is an exponent relevant to the coarsening mechanism.

Three kinetic models were developed to explain the OA-based crystal growth of nanocrystals based on their diameter [69, 70]. These models can be categorized based on the collision between two primary nanoparticles (A1 + A1 model) or a primary particle and a multilevel particle (A1 + Ai model) or two multilevel particles ((Ai + Aj) multistep kinetic model), as shown in **Figure 4(a)**–**(c)**, respectively. The aggregation of nanocrystals using the OA mechanism is described in these models using the time evolution size distribution of nanocrystals, based on the Smoluchowski Equation [69]. These population growth matrixes of OA kinetic models account for nanostructures with different diameter sizes. The simplest growth model is A1 + A1 primary particle model and it can be stated in Eq. (2) [71].

$$d = \frac{d\_0 \left(\sqrt[3]{2}k\_1 t + 1\right)}{\left(k\_1 t + 1\right)}\tag{2}$$

where *d*0 is the mean diameter at time *t* = 0 (primary nanoparticle), *d* is the mean diameter at time *t* (secondary nanoparticle).

The prior arts of these existing kinetic models that were used to understand the crystal growth mechanism by fitting the experimental observation of synthesis of different nanocrystals have summarized in the following **Table 1**. As we can see in these studies, most often, both OR and OA mechanisms occur simultaneously or coexist in the same model. Zhang and coworkers found that the solo OA mechanism causes to grow the nanocrystals by hindering OR mechanism at the initial stages by introducing surfactants to strongly adsorb them onto crystal surfaces [66, 74]. Moreover, Zhuang et al. observed that the OA mechanism become dominated under unsaturated solutions by suggesting that OR mechanism can be thermodynamically prohibited without having enough concentration gradient to dissolve nanoparticles in a surfactant-free hydrothermal synthesis route [76].

These state of the art of OA-based nanostructures show that all the existing kinetic studies have been performed based on the nanoparticle's diameter growth using the modified Smoluchowski equation. In addition, two reports have attempted to explain the kinetic rate for the elongation of 1 D nanorods by considering dipole attraction for their alignment [77, 84]. The in-depth understanding of the kinetics of crystal growth mechanisms for the fabrication of 1 D nanostructures is still in its preliminary stage although there is rapid progress in fabricating semiconductor nanowires. Therefore, it is vital to develop new kinetic models for

#### **Figure 4.**

*The illustration of (a) A1 + A1 model, (b) A1 + Ai model, and (c) Ai + Aj model (A1 is a primary particle and Ai and Aj are multilevel particles; n = 1, 2, 3, 4, ….).*

*Ultrathin Metal Hydroxide/Oxide Nanowires: Crystal Growth, Self-Assembly, and Fabrication… DOI: http://dx.doi.org/10.5772/intechopen.101117*


#### **Table 1.**

*Summary of crystal growth kinetics of different metal/metal oxide nanocrystals and nanostructures studied from 2003 to 2019.*

1 D nanostructures with directing their length for the elevation of the controlled fabrication of ultrathin metal hydroxide/oxide nanowires.

## **3. Greener synthesis approaches for fabricating ultrathin metal hydroxide/oxide nanowires for optoelectronic applications**

Due to the remarkable physical properties of 1 D transition metal hydroxide/ oxide nanostructures, greener fabrication of anisotropic metal hydroxide/oxide nanowires has attracted increasing attention in many applications. Yang et al. summarized the recent efforts of controlled synthesis of metal oxide and hydroxide 1 D nanostructures such as NiO nanorods and Co3O4 nanowires via hydrothermal route for high-performance electrochemical electrodes and catalysts [19]. A wet chemical synthesis method was demonstrated the fabrication of ZnO nanorods, with a diameter of ~15 nm by oriented attachment mechanism [86]. Very recently, our group introduced a versatile sol–gel synthesis combined with the solvothermal process to make ZnO nanorods for optoelectronic devices [87]. Ultrathin ZnO nanorods with a diameter of ~7 nm were fabricated using a modified solvothermal route [88]. These nanorods show a strong UV band edge emission suggesting the applications of photoelectric nanodevices. Furthermore, Chaurasiya et al. synthesized TiO2 nanorods using a wet chemical method for photovoltaic and humidity sensing applications [89], while MnO2 nanorods were fabricated using a hydrothermal synthesis method for supercapacitor applications [90]. The work of hydrothermally growth GaOOH nanorods in width of 200–500 nm presented a low-cost and large-scale production strategy to prepare nanorods for practical applications [91].

Many works have been reported for the greener synthesis of 1 D Cu(OH)2 and CuO nanostructures [92–96]. However, only a very few reports have demonstrated cost-effective and efficient synthesis routes to fabricate ultrathin copper hydroxide/ oxide nanowires. In previous reports, ultrathin Cu(OH)2/CuO NWs were synthesized using either both weak and strong bases [97] (i.e., aqueous ammonia and NaOH/KOH solutions) or by the interaction between a copper complex and NaOH at the aqueous-organic interface [98]. Sundar et al. demonstrated a bio-surfactant assisted synthesis method to produce CuO nanowires, suggesting the crystal growth process follows the oriented attachment mechanism [99].

Very recently, our group fabricated scalable and reproducible ultrathin CuO nanowires from self-assembled ultrathin Cu(OH)2 nanowires, using a facile, greener and surfactant-free sol–gel approach, as shown in **Figure 5** [100]. As depicted in **Figure 6**, we observed the nanocrystals self-assembly into a certain crystallographic orientation and after 45 min stirring it forms smoother surfaces of colloidal nanowires with a uniform diameter of 6 ± 2 nm from 1 hour to 4 hours stirring time intervals. The time-dependent X-ray diffractometer (XRD) analysis is supported to identify the crystal growth plane of nanowires along the [020] facet. Upon annealing the ultrathin Cu(OH)2 nanowires on the Si substrate at 300°C for 1 hour, we fabricated high aspect ratio CuO nanowires over a large area, as shown in **Figure 7**. The respective XRD and XPS spectroscopies confirm the chemical composition of both Cu(OH)2 and CuO nanowires and their purity. The calculated optical band gaps for Cu(OH)2 and CuO nanowires are 1.51 and 1.10 eV, respectively.

Wang et al. fabricated uniform ultrafine Cu(OH)2 and CuO nanowires using a simple wet chemical route for lithium-ion batteries [101]. Another study reported the aqueous phase synthesis of Cu(OH)2 nanowires with diameters of about 10–20 nm [102]. They have initiated to study the OA crystal growth mechanism of Cu(OH)2

**Figure 5.**

*Reaction scheme for the synthesis of ultrathin Cu(OH)2 and CuO nanowires using sol–gel hydrolysis followed by directed self-assembly and annealing.*

#### **Figure 6.**

*Time-dependent TEM images at different stirring time intervals after the addition of NaOH during the synthesis of Cu(OH)2 NWs (re-created from the original data).*

*Ultrathin Metal Hydroxide/Oxide Nanowires: Crystal Growth, Self-Assembly, and Fabrication… DOI: http://dx.doi.org/10.5772/intechopen.101117*

**Figure 7.** *SEM image of CuO nanowires fabricated on Si-substrate (re-created from the original data).*

NWs, utilizing the power of high-resolution transmission electron microscopy (HRTEM) analysis. Very recently, Bhusari et al. reported the sol–gel approach to preparing Cu(OH)2 NWs with a maximum diameter of 30 nm by varying the pH and temperature of the solution [103]. However, there is no experimental work reported for in detail investigations of their crystal growth mechanism, reproducibility, scalability and optimization of reaction parameters to get desired dimensions of nanowires for real applications in optoelectronics.
