**Abstract**

One-dimensional (1D) inorganic metal/metal oxide nanostructures are of significant interest due to their distinctive physical and chemical properties that are beneficial for various applications. A fundamental understanding of the guiding principles that control the anisotropy and the size of the nanostructures is essential toward developing the building blocks for the fabrication of leading-edge miniaturized devices. Oriented attachment (OA) crystal growth mechanism has been recognized as an effective mechanism for producing 1D anisotropic nanostructures. However, a limited understanding of the OA mechanism could impede the controlled fabrication of 1D nanostructures. This chapter provides a comprehensive summary on recent advances of the OA mechanism and the current state of the art on various *in-situ, ex-situ*, and theoretical investigations of OA-based crystal growth dynamics as well as the shape and size-controlled kinetics. Other competing crystal growth mechanisms, including seed-mediated growth and Ostwald ripening (OR), are also described. Further, we thoroughly discuss the knowledge gap in current OA kinetic models and the necessity of new kinetic models to elucidate the elongation growth of anisotropic nanostructures. Finally, we provide the current limitations, challenges for the understanding of crystal growth dynamics, and future perspectives to amplify the contributions for the controlled self-assembled 1D nanostructures. This chapter will lay the foundation toward designing novel complex anisotropic materials for future smart devices.

**Keywords:** one-dimensional nanostructures, anisotropy, oriented attachment mechanism, crystal growth dynamics, kinetic models, seed-mediated growth, Ostwald ripening

### **1. Introduction**

#### **1.1 Anisotropic one-dimensional nanomaterials**

The controlled fabrication of crystalline anisotropic nanostructures has flourished to obtain unique functional properties and characteristics arising from their quantum confinement and nanoscale size effect for many different industrial applications. For decades, the structure-property correlations and their corresponding mechanistic principles of formation were extensively elucidated. The guiding principles and factors that affect size- and shape-control of a material, including crystal growth mechanism, phase transformation, and kinetics, have been explored for anisotropic onedimensional (1D) nanostructures, such as nanorods, nanotubes, and nanowires [1–4]. The 1D electronic pathways to accumulate effective charge transportation and larger surface area of anisotropic nanostructures provide a profound impact in nanoelectronics and nanodevices [5]. Therefore, 1D transition metal/metal oxides crystalline nanomaterials are critical building blocks for the next-generation highperformance integrated circuits and Internet of things (IoT) applications [6–9].

Different *in-situ* interpretations have been reported to provide insights into the crystal growth of nanomaterials and how the morphology is controlled. Liquid-phase atomic force microscopy (AFM), [10, 11] cryo-transmission electron microscopy (Cryo-TEM), [12] liquid-phase TEM, [13, 14] field emission scanning electron microscope (FE-SEM), [15] and time-resolved small-angle X-ray scattering (SAXS) [12] are emergent techniques that reveal the real-space imaging of nanocrystals, intermediate structure formation, crystallization, and their growth kinetics. The efforts on in-depth investigations to understand the factors and driving forces that control the anisotropy of a nanomaterial at an atomic scale have evolved and are continuously expanding.

The significant advances of bottom-up synthetic routes that produce nanomaterials with various morphologies have been devoted to obtaining their distinctive electronic, optical, mechanical, catalytic properties. While different synthetic procedures are capable of fabricating 1D nanostructures, wet chemical methods satisfy the scalability and lower the cost. However, the development of cost-effective, greener synthesis methods that are scalable and reproducible to make stable 1D nanostructures still needs to be addressed. To accomplish this task, the foundational investigations on crystal growth mechanisms, kinetics, and phase transformation to fabricate 1D anisotropic nanomaterials are crucial. The real-time nonclassical crystallization dynamics of 1D nanostructures in solution-based synthetic processes at different temperatures and a high magnification have been limited and remain a challenge. We begin this chapter by describing the mechanisms of OA and provide insights into the OA crystal growth kinetics of anisotropic 1D nanostructures. Then we provide the recent progress of OA-based 1D metal/metal oxide nanostructures and other competing crystal growth mechanisms to lay the foundation for OA-directed anisotropic crystal growth process. We subsequently provide an insight into the characterization techniques for various *in-situ* and *ex-situ* investigation of OA-based crystal growth dynamics that makes anisotropic nanostructures along with the current state of the art. We further elaborate the theoretical studies developed along with the experimental techniques that enhance the understanding of the OA-based crystal growth mechanism and morphology evaluation. Then, we present a time-dependent crystal growth observation process of OA-based crystal growth of anisotropic copper hydroxide nanowires formed in a sol-gel colloidal system with an in-depth discussion of its growth kinetics, correlating to the sol-gel chemical kinetic reactions. Finally, we *Oriented Attachment Crystal Growth Dynamics of Anisotropic One-dimensional Metal/Metal… DOI: http://dx.doi.org/10.5772/intechopen.107463*

provide future perspectives of direct observation of crystal growth dynamics that enhance the fundamental understanding of nanoscale colloidal assembly mechanisms to achieve morphology-controlled properties of nanomaterials for future needs in advanced applications.

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

In recent years, nonclassical oriented attachment (OA) mechanism has received a great attention to produce various morphologies including zero-, one-, two-, and three-dimensional nanostructures with their controlled structural properties [16]. More importantly, it is an effective mechanism for anisotropic crystal growth of nanostructures, inclusion of defects to the crystal structure, and the formation of branched nanostructures and highly ordered mesocrystal structures [17]. This key crystal growth mechanism can be found in natural biomineralization processes that show unique physicochemical properties [18]. In 1998, the observation of anisotropic chain of TiO2 anatase nanocrystals attachment across the {112} crystal facets using a high-resolution transmission electron microscope (HR-TEM) by Penn and Banfield had contributed to this discovery of the OA mechanism [19]. The thermodynamic reduction in the surface energy of TiO2 crystal facets during the attachment process has driven the structural anisotropy [19, 20].

#### *1.2.1 Concept and mechanism of OA*

OA is the self-assembly of adjacent nanocrystals for a specific crystal facet attachment to form a secondary single nanocrystal through the Brownian motion [19, 21]. While Penn and Banfield visually observed the OA-based crystal growth using TEM, Moldovan and coworkers proposed the detailed mechanistic principles in 2002 using both analytical and molecular-dynamics (MD) simulation methods [22, 23]. According to their model named as "Grain-Rotation-Induced Grain Coalescence (GRIGC) mechanism," the adjacent primary nanocrystal colloids in a solution freely rotate to match the perfect coherent grain-grain interface. After they met perfect crystal facet match, these nanocrystals start coalescence to eliminate the common grain boundaries and form a single large nanocrystal [22]. This is the thermodynamic reduction of the crystal surface energy of nearby nanocrystals that drive to minimize the high energy surfaces. **Figure 1** shows the schematic representation of GRIGC mechanism to form an OA-based 1D nanostructures such as nanorods and nanowires.

#### *1.2.2 Characteristic of OA*

The anisotropic nanostructures produced using OA process provide unique characteristics in size, crystal structure, and kinetics. The constant diameter during the growth is one of the unique advantages of OA mechanism by attaching of nanocrystals to the tip of the growing 1D nanostructure [4]. This is very similar to the polymerization reaction processes. Also, there is a capability to predetermine the diameter of the nanostructure, by observing the diameter of the monodispersed nanocrystals. If the diameter of 1D nanostructure is below 10 nm, it offers interesting and improved characteristics. For example, ultrathin nanowires provide new surface determined structures, with tunable surface chemistries, higher surface area, and colloidal stability for different applications [24–27]. Nanocrystal that grows following the OA mechanism has abundant defects such as twin planes, stacking faults, and misorientations

#### **Figure 1.**

*Schematic representation of stages on OA crystal growth mechanism to form an anisotropic nanostructure. (a) Primary nanocrystal colloids in a solution, (b) Rotation of nanocrystals to match the crystal facets of nanocrystals, (c) Coalescence of nanocrystals along a specific crystallographic orientation, and (d) Formation of a single crystal 1D nanostructure.*

[28, 29]. This defect formation occurs due to the crystal lattice mismatch during the nanocrystal attachment process.

OA-based chemical reactions follow second-order kinetics, and there are three kinetic models developed to explain the 1D crystal growth kinetics of nanocrystals based on collision of nanocrystal numbers [2]. These existing kinetic models were developed based on the diameter growth of the nanocrystals. The crystal growth rate is correlated exponentially with the nanocrystals'surface energies in OA growth [30]. The crystal growth rate is higher in high surface energy planes; therefore, the final crystal facet of the product is the lower than the surface energy crystal plane. The morphology or the size of OA-based 1D nanostructures can be modulated by preferentially adsorbing solvents, ligands, and surfactants. They adsorb selectively with different nanocrystal binding affinities onto specific crystal facets [31]. Studies of shape-controlled synthesis of different 1D nanostructures using selective adsorption of surfactants have been reported [32, 33].

#### *1.2.3 Recent progress of OA-based metal/metal oxide nanostructures*

To date, OA mechanism has been used to fabricate different metal/metal oxides/ metal hydroxides 1D nanostructures to use in different applications. Few recent examples of controlled synthesis of different nanorods and nanowires are Au [34], NiO [35], Sb2O3 [36], Co3O4 [35], ZnO [37–39], TiO2 [40], MnO2 [41],CuO [42, 43], Cu (OH)2 [44], and GaOOH [45] for optoelectronic devices, electrochemical devices, and supercapacitors. Although these metal/metal oxide nanostructures have been cited as OA mechanism-based nanostructures, only few studies have investigated the crystal growth mechanism with the understanding of their guiding principles. The very first studies of Penn and Banfield investigated imperfect attachment of anatase TiO2 nanocrystals by describing their driving tools of nanocrystals rotation and coalescence observed by HRTEM [19]. Recently, Zhang and coworkers demonstrated direction specific van der Waals attractions between two rutile TiO2 nanocrystals [46]. They utilized AFM probe technology with environmental TEM (ETEM) to elucidate the relationship between the orientation, contact area, and surface

*Oriented Attachment Crystal Growth Dynamics of Anisotropic One-dimensional Metal/Metal… DOI: http://dx.doi.org/10.5772/intechopen.107463*

roughness of nanocrystals. Another outstanding investigation of OA process is the direct observation of iron oxyhydroxide nanoparticles by liquid cell HR-TEM [47]. This study reveals the importance of electrostatic interactions for the lattice match attachment of nanocrystals, and they have successfully attempted to measure the translational and rotational accelerations. Leite *et al* performed an *ex-situ* observation of the OA crystal growth process of SnO2 nanocrystals at room temperature using HR-TEM [48]. Another example of investigating the driving forces of OA process is ZnO nanocrystals using the HR- TEM coupled with x-ray diffraction (XRD) in the gas phase in the presence of a constant electric field [49]. They suggested that the OA process was dominant due to the increased dipole interactions of ZnO nanocrystals with the electric field. The recent investigation of OA-based Au nanocrystals using liquid cell-TEM provided evidence of the control of crystal facets by capping ligands to adsorb on different surfaces at an atomic scale [14]. The real-time observation of nanobridge formation between adjacent Au nanocrystals and then rearrangement of single nanocrystal via grain boundary migration using liquid cell-TEM is a remarkable investigation of OA process, which corresponds to the self-assembly of nanocrystals via "jump-to-contact" mechanism [50].

#### **1.3 Other crystal growth mechanisms**

#### *1.3.1 Seed-mediated growth*

Seed-mediated growth is a common growth mechanism to produce well-controlled crystalline noble metal nanostructures. It involves two main steps: nucleation and growth, as shown in **Figure 2**. In nucleation, the metal precursors undergo the reduction process to form zerovalent metal atoms that self-assemble into small clusters to further grow into stable nuclei. These crystalline nuclei act as monodispersed seeds for the subsequent growth of metal nanostructures during the growth stage [51]. This mechanism can be divided into two main types based on their temporal and spatial differences of the nucleation and growth stages, which includes homogeneous and heterogeneous nucleation [52]. In homogeneous nucleation, the seed nanocrystals are generated and followed by nucleation and growth processes in the same chemical reaction. In contrast, the seed nanocrystals are synthesized separately and then added into a growth solution to further growth of nanocrystals in the heterogeneous nucleation.

The surfactants/capping agents such as cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC) can facilitate the controlled crystal facet directed growth of nanostructures via preferential adsorption. Although this is a very

**Figure 2.**

*Schematic representation of seed-mediated growth to form an anisotropic nanostructure.*

versatile process, the major drawback of this mechanism is the limited length growth of nanostructures due to the weak interaction forces [9]. Early attempts to produce Au nanorods designed a three-step seeding protocol by Murphy and Obare team [53]. **Figure 3** shows the different aspect ratio of synthesized Au nanorods at different pH values in the solution. This modified seed-mediated method produced longer nanorods at higher pH value with the high yield of product.
