**3. Novel approaches to understand OA longitudinal growth dynamics and kinetics of anisotropic 1D nanostructures**

One-dimensional transition metal/metal hydroxides/oxide nanostructures are ideal building blocks for the miniaturization of devices. Understanding the mechanism and kinetics that control the final morphology of nanostructures is essential for the progress toward materials design for desired applications. Early studies show that surfactants/organics are one of the guiding factors for the OA growth. However, the presence of organics is not compulsory to drive the OA mechanism [96]. There are

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

enough examples to demonstrate OA-based nanostructure formation without any organic additives [37, 97, 98]. The temperature and pH value of the reaction are other driving forces for OA mechanism [99]. However, the attempts observing the mechanism to elucidate the kinetics of OA-based elongation of 1D nanostructures are still under studied. Few reports that attempted to explain the crystal growth kinetics of one-dimensional nanostructures along certain specific direction are described in this section.

Gunning and the team presented the multistep kinetic model to explain the elongation of CdS nanorods using dipole-dipole interactions in the presence of amine [100]. They suggested that the amine concentration drives the end-to-end attachment of nanocrystals by minimizing the surface energy to align the nanocrystals. Another study investigated the roles of van der Waals attractions and Coulomb interactions for OA growth of nanorods and nanowires [101]. These 1D nanostructures are assumed to be cylindrical shape and nanocrystals to be spherical shape. The Hamaker's particleparticle model was used to calculate van der Waals attractions in the system to explore the effect of head-to-head attachment of nanorods. It was found that the role of van der Waals interaction was to guide the 1D nanostructures formation. Furthermore, they calculated the role of Coulomb interactions for OA growth using Coulomb's law. They have considered the nanoparticle-nanorod separation to find out the effect of different parameters on Coulomb-interactions-based OA growth. He and coworkers also investigated the parameters associated with van der Waals interactions to drive the OA growth for nanorod formation [102].

Our group achieved a significant milestone by developing novel chemical kinetic models for the first time to describe the OA-directed crystal growth kinetics in the solgel chemical process [103]. By assuming that the sol-gel process is a quasihomogeneous system, dimensional changes in the nanocrystals at three stages of the sol-gel process that forms Cu(OH)2 nanowires were monitored using *ex-situ* TEM. It was found out that nanowire growth follows second-order, zeroth-order, and zerothorder sigmoidal Boltzmann models during the hydrolysis and condensation, first stage of polycondensation, and the second stage of polycondensation phases, respectively, as shown in **Figure 6**. The sigmoidal growth curve represents three characteristic phases including the initial lag phase, growth phase, and saturation phase that correspond to the three stages of the sol-gel process. The first stage of hydrolysis and

#### **Figure 6.**

*The sigmoidal plot that describes three stages in the sol-gel process to fabricate ultrathin Cu(OH)2 nanowires as described in reference [103].*

condensation process follows second-order kinetics to the self-organization of nanocrystals to form nanochains. Then the directional elongation of nanoarrays along a specific crystal facet occurs during the first stage of polycondensation following zeroth-order kinetics. Finally, the second phase of polycondensation demonstrated a steady saturated growth that forms Cu(OH)2 nanowires following zeroth-order kinetics. To validate these kinetic models, we utilized statistically significant datasets, higher the regression coefficient (R2 ) values, which is at 95% levels of confidence for three replicates.

We identified these important stages of OA mechanism, including nanocrystal formation and their self-assembly toward orientation, nanoarrays, and nanowires formation by observing time dependent *ex-situ* TEM images. **Figure 7** demonstrates the formation of primary Cu(OH)2 nanocrystals (sols) during the hydrolysis and condensation. We successfully captured the neck initiation between two nanoparticles, which shows same crystallographic orientation. Similar observations were reported by Wang *et al* for the PbSe nanocrystals using *in-situ* liquid cell TEM [104]. The crystallographic lattice orientation and the epitaxial growth nanocrystals were determined by the (Fast Fourier Transform) FFT and SAED, respectively. The dspacing of a nanocrystal confirms the Cu(OH)2 crystal structure.

The HR-TEM image in **Figure 8a** shows the alignment of nanocrystals forming nanocrystal chain after 5 min of stirring. The coalescence of nanocrystals is incomplete, and lattice orientations rotation is visible toward the same crystal facet orientation. Therefore, our study further confirms the evidence of continuous fusion and rotation of different crystal-faceted nanocrystals to match their lattice orientation in the OA mechanism. The next image in **Figure 8b** shows the progress of nanocrystals orientation with smoothing the edges after 1 hour stirring time. The formation of nanowire after 4 hours stirring is shown in **Figure 8c**, and we can observe neck elimination between nanocrystals and a uniform distribution of lattice orientation in this nanowire. The inset FFT pattern confirms the atomically coherent single crystal, which is the final product of the OA mechanism. This study provides insight to produce anisotropic

#### **Figure 7.**

*(a) TEM image of nanocrystal seed formation during the addition of base, (b) HR-TEM image of Cu(OH)2 nanocrystals with respective lattice d-spacing for [022] facet; arrow marks indicate crystal lattice orientation of different nanocrystals, (c) respective FFT image, and (d) its SAED pattern. Reproduced with permission from reference [103] Copyright {2022} American Chemical Society.*

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

#### **Figure 8.**

*HR-TEM images of nanoarrays at: (a) 5 min, (b) 1 hr, (c) 4 h stirring time, respectively. (Inset is its FFT, and the red arrows indicate necks between nanocrystals) Reproduced with permission from reference [103] Copyright {2022} American Chemical Society.*

nanostructures of other metal oxides via the sol-gel processes by tailoring the reaction parameters, such as reaction time, temperature, solvent, and the pH.

#### **4. Summary, challenges, and future prospective**

Inorganic one-dimensional metal/metal oxide nanostructures play important roles in various miniaturized electrical, optical, and energy devices. The exploration of nanostructure-property relationship in terms of size, shape, interaction, and crystal growth of nanostructures is essential for the rational design and synthesis of tailored anisotropic nanostructures. However, the reaction kinetics and crystal growth mechanisms are not well understood, and the further development of both theories and experiments is expected. The OA mechanism, which is the effective mechanism to fabricate anisotropic structures, requires understanding their guiding factors that enable the rational design of different 1D nanomaterials. The driving forces for the preferred attachment orientations, origin of the force field between nanocrystals, collision trajectories, surface energies of crystal facets are needed further investigations, using direct *in-situ* observations, *ex-situ* methods, and computer simulations. Although there is a development of experimental and theoretical investigations of OA mechanism for 1D nanostructure fabrication, certain areas still need to be addressed for the development of next-generation high-performance devices.

### **4.1 Limitations of** *in-situ* **experimental techniques to investigate guiding factors at an atomic scale**

The recent advances in the *in-situ* experimental techniques facilitate the direct real-time observations of the OA crystal growth. The different key factors may be synergistic in driving OA growth successfully in a complex environment. Therefore, identifying individual contributions and their significance is challenging. *In-situ* techniques allow us to track the trajectory of nanostructures such as nucleation, nanocrystals formation, self-assembly, coalescence and growth, and the physical behavior in a solution. These facilities are not available and limited in *ex-situ* experiments.

The development of liquid cell TEM has led to make it capable to visualize at an atomic scale and has contributed significantly for the reasonable growth in this field. However, there are limitations and challenges during *in-situ* imaging of nanomaterials. The electron beam irradiation effect, localized heating effects that create artifacts or defects in the sample, substrate effects are common problems in liquid cell TEM. Currently, these effects can be mitigated to some extent by controlling the beam exposure, flux, and current, to reduce the electron beam effects and radiation damage of nanostructures and introduction of different sample stages to promote heat dissipation during imaging. Some other potential solutions are introduction of high detection efficiency camera systems and couple compressive sensing software system into the TEM. However, there are still more unstable materials yet to be explored by *in-situ* imaging as they are more sensitive to the electron beam. It is required to optimize the parameters in the TEM to reduce the beam exposure and damages with different materials. Although this is very time-consuming and complicated technique, it is worth to generate information using different nanomaterials that enhances our fundamental understanding on OA mechanism. Another challenge of *in-situ* imaging is having complicated synthesis procedures to fabricate metal/metal oxide 1D nanostructures. The new facile and efficient solution route-based synthesis procedures need to be in place as they are more convenient to investigate fundamental principles using *in-situ* experiments. The rapid progress and introduction of new features to TEM or other instruments are also advancing these investigations.

### **4.2 The invalid existing OA kinetic models to describe the elongational crystal growth of anisotropic structures**

The existing modified Smoluchowski kinetic models can only describe the diameter growth in the OA mechanism. The use of these kinetic models for different nanocrystals is very limited and understudied. The kinetic studies of OA-directed crystal growth processes are helpful to address the development of different synthesis methods of size and shape-controlled 1D nanostructures. However, there is less development of kinetic models to describe the elongation growth of 1D nanostructures. Although few studies are attempted to describe the OA elongation kinetics, they are limited to their specific system and haven't modeled using experimental observations. The theoretical kinetic models are essential to develop, introduce, and validate with multiple experimental observations. It could further expand to *Oriented Attachment Crystal Growth Dynamics of Anisotropic One-dimensional Metal/Metal… DOI: http://dx.doi.org/10.5772/intechopen.107463*

describe the OA crystal growth processes in the presence of surfactants or organic additives.
