**1. Introduction**

#### **1.1 Transition metal oxides nanowires**

Transition metal oxides nanowires are known as an important class of materials with a rich collection of physical and chemical properties due to their superior performances based on quantum confinement effects for various general applications, including electronic devices, optical devices, gas sensors, photovoltaics, photonic devices, energy storage devices, and catalysts [1–5]. The fabrication of one-dimensional (1 D) nanostructures of transition metal oxides semiconductor

(TMOS) nanowires and nanorods has been recently fascinating for the next-generation high-performance "trillion sensor electronics" era for Internet of things (IoT) applications, mainly due to their high surface to volume ratio, high crystallinity, and low power consumption [6–10]. The ultrathin nanowires that have below 10 nm diameter offer interesting characteristics including new surface determined structures by tuning the surface chemistry of surfaces [11, 12]. Furthermore, high surface area and increased colloidal stability are also inherited to ultrathin nanowires that are related to the decreased diameter of nanowires [11]. These improved overall properties of ultrathin TMOS nanowires as an ideal building block will continue the miniaturization and functional scaling of integrated circuits (ICs), nanoelectronics and optoelectronic devices by achieving the limitations of Moore's law.

A significant research endeavor has been devoted to the fabrication of 1D metal hydroxide/oxide nanowires. However, their controlled fabrication to tailor the shape, size, crystallinity, and anisotropy is remaining a challenge. Tremendous efforts are needed to devote to the development of effective, greener fabrication methods that has scalability, reproducibility, and stability of nanowires for the successful commercialization and integration of devices. The fundamental in-depth understanding of guiding principles such as crystal growth mechanisms, kinetics, and phase transformation during the fabrication of metal-hydroxide/oxide nanowires with different strategies is crucial to accomplish this goal for promoting TMOS nanowirebased optoelectronic devices. In this review, first, we give a comprehensive overview of different synthesis strategies of transition metal-oxide/hydroxide nanowires and their electrical and optical properties. Then we provide the fundamentals of crystal growth mechanisms and a detailed overview of oriented attachment (OA), crystal growth mechanism that makes anisotropic nanowires. We subsequently discuss the state-of-the-art kinetic models that explain the OA crystal growth mechanism. Then we present a greener facile synthesis approach for the fabrication of ultrathin copper hydroxide/oxide nanowires and their optoelectronic properties. Finally, we provide an outlook of the challenges of future prospective to fabricate ultrathin transition metal hydroxide/oxide nanowires for optoelectronic applications.

#### **1.2 Synthesis strategies of transition metal oxide nanowires**

The bottom-up techniques are prominent to produce transition metal oxide NWs due to the high purity of the product, low-cost fabrication, and dimension controllability [13]. The controlled fabrication of transition metal hydroxide/oxide nanowires can be done with either vapor or solution phase growth strategies. Shen and coworkers have summarized different 1-D metal oxide nanostructures including nanowires, nanobelts, nanorods and nanotubes synthesized using both vapor or solution phase growth strategies [14]. However, the controlled pressure of the inert atmosphere and high-temperature vapor-phase approaches including vapor phase growth such as vapor–liquid–solid (VLS), solution–liquid–solid (SLS), vapor–solid–solid (VSS) or vapor–solid (VS) process, physical vapor deposition (PVD) and chemical vapor deposition (CVD) are expensive and need sophisticated instruments [15–18]. Therefore, solution-based wet chemical strategies such as hydro-thermal method, thermal decomposition, electrochemical methods, solvothermal methods, sol–gel routes became popular as they are inexpensive, energy savers, excellent control over size and morphologies, with ease of larger-scale production [19].

The wet chemical routes can be performed through precipitation or oxidation of the precursor by the aid of catalyst or surfactants and the aid of heating in an oxygen-rich environment to make metal oxide/hydroxide NWs [20]. In a hydrothermal route, heating the precursor solution/substrate and then annealing in an oxygen environment is required to form metal hydroxide/oxide nanowires [19].

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

Solvothermal methods are performed by heating a metal precursor solution to a high temperature with the presence of a solvent [21]. Microwave-assisted methods are another powerful approach that require heating to a higher temperature in a microwave [22]. However, these most wet chemical growth strategies require expensive chemicals, heating procedures, longer reaction times or templates or impurities that needs to remove after the procedure [20].

The sol–gel method is a green and low-temperature method, which is widely employed to make homogeneous, highly stoichiometric and high-quality metal oxide/ hydroxide nanowires in a larger scale production [23, 24]. It is often used to fabricate size and shape-controlled nanostructures starting from a metal salt as the precursor and catalyzed by base or acid to form an integrated network or gel. This method is popular to make different solid networks such as inorganic hydroxides/oxides, organic polymers, organic–inorganic hybrids, and composites due to its advantages such as high yield, better reproducibility, low operation temperature and low-cost method of highly stoichiometric and homogeneous products [25–27]. The sol–gel process can be defined as the transition of a liquid solution "sol" into a solid "gel" phase, involving both physical and chemical reactions such as hydrolysis, condensation, drying, and densification, as shown in **Figure 1** [28]. When preparing metal hydroxides/oxides, first, the sol is formed from the hydrolysis of metal precursors. Sol is a stable dispersion of colloidal particles in a liquid solution and particles can be amorphous or crystalline. Then the gel is formed through condensation, polycondensation and aging to form a gel network using metal–oxo–metal or metal–hydroxy–metal coordinate bonds. Drying and densification involve densifying the gel by collapsing the porous gel network.

The morphology is influenced either due to different surface energies of the crystal faces or the external growth environment, which combines with different factors such as the precursor to base concentration, solvent polarity, temperature, and crystal growth mechanism. Different types of metal oxides have been synthesized using sol– gel route, exhibiting different 1D morphologies such as nanorods, nanoplatelets, and wires [29–32]. The conventional sol–gel process is hydrolytic, as shown in **Figure 2** and oxo ions are originated from water in the reaction medium. When using organic solvents as reagents in the medium, they are nonhydrolytic sol–gel process pathways and the oxygen atoms are originated from the organic O-donor [25, 33–35]. However, both sol–gel processes have their limitations with different metal precursors [25, 35].

## **1.3 Electronic and optical properties of transition metal oxides**

Transition metal oxides offer very diverse and fascinating electrical and optical properties of materials, which arise from the outer d electrons of the transition metal ions. Many transition metals can form binary oxides of the formula MxOy. The range of electrical conductivity of transition metal oxides is wide and varied from metals to semiconductors and insulators. A few examples of transition metal oxide insulators are CaO, NiO, TiO2 and the semiconducting materials are FeO, ZnO, and CuO, while TiO, NbO, CrO2, ReO3 show the metallic properties

**Figure 1.** *Steps of a typical sol–gel process.*

#### **Figure 2.**

*The formation of metal hydroxides/oxides via hydrolytic and nonhydrolytic sol–gel process.*

respectively [36]. Every *3d* transition metals monoxides are widely used in different applications due to their higher abundance and low cost compared to *4d* and *5d* transition metal oxides.

Among transition metal oxides, copper oxide has received considerable attention in recent years as an alternative element for expensive silver and gold due to its second-highest electrical conductivity and higher abundance [37]. Cupric oxide (CuO) is a p-type semiconductor with a narrow and indirect energy bandgap of ∼1.2 eV [36]. Copper hydroxide (Cu(OH)2 is the hydroxide form, having an indirect bandgap of 1.97 eV [38, 39]. Zinc oxide (ZnO) is also a widely used and studied material, which is composed of the next element to Cu in the periodic table. It is an n-type semiconductor with a direct wide bandgap of 3.3 eV [40]. The electronic properties have been widely studied for CuO and ZnO nanostructures [41–43]. By fabricating smoother surfaces with minimum defects of metal oxide dielectrics can utilize their good electrical insulation without compensating its high *k* [44]. Since metal oxides are typically wide bandgap materials, they show excellent optical properties. For example, ZnO nanostructures have been mainly reported for laser diodes and LEDs that is the potential to operate at room temperature, owing to their higher exciton binding energy [45–47]. Ye Zhao et al. reported the optical properties of MoO3, exhibiting a wide optical band gap of ∼3.05 eV [48]. The controlled hydrothermal synthesis of ZrO2 1D nanostructures has shown optical properties, which are suitable in light-emitting devices [49]. In addition, CuO, NiO, SnO, and Ta2O5 are also have shown optoelectronic properties [10, 50–53].
