Oriented Attachment Crystal Growth Dynamics of Anisotropic One-dimensional Metal/Metal Oxide Nanostructures: Mechanism, Evidence, and Challenges

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.

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 one-dimensional (1D) nanostructures, such as nanorods, nanotubes, and nanowires [1, 2, 3, 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 high-performance integrated circuits and Internet of things (IoT) applications [6, 7, 8, 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 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.

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.

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, 25, 26, 27]. Nanocrystal that grows following the OA mechanism has abundant defects such as twin planes, stacking faults, and misorientations [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, 38, 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 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.

Figure 2.

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

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 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.

Figure 3.

TEM images of seed-mediated growth-assisted Au nanorods after one round of purification having (a) aspect ratio ∼18 at pH = 3.5, and (b) aspect ratio ∼25 at pH = 5.6. These images were reproduced from Brantley et al. [53] with permission from Willey Online Library.

1.3.2 Ostwald ripening (OR) mechanism

The classical crystal growth mechanism is the Ostwald Ripening (OR) that describes the smaller crystals grow into the larger crystals through the diffusion to reduce the total surface free energy (Figure 4) [54, 55, 56]. It is a concentration gradient-induced process around the surface of particles that follows the Gibbs-Thompson relation, as shown in Eq. (1) [56, 57]. Therefore, it results in spherical particles most often, which are in the micrometer size diameter range. However, this mechanism is most often unable to describe the crystal growth in the nanoscale. Further, the particles with similar crystal symmetries can also obtained due to the similar crystal facet surface energies [56].

Figure 4.

Schematic representation of OR-based crystal growth.

$$C_r=C_e\mathit{exp}\left[\frac{2\mathit{\gamma \Omega }}{\mathit{RTr}}\right]$$E1

where $C_r$ is the equilibrium concentration at the surface of the particle, $C_e$ is the equilibrium concentration at a plane interface, $\gamma$ is the surface free energy, R is the universal gas constant,$\Omega$ is the molar volume of the particle, T is the temperature, and r is the radius of the particle.

Since there is no involvement of crystal facets between the neighboring crystals, in the OR process the crystal defects are less than OA based structures. Although OA and OR the growth processes involve the growth of nanocrystals, the size gradually increases in OR mechanism, while particle size increment is stepwise in OA. Most often, both OR and OA mechanisms involved during the many synthesis processes and determine the final morphology.

1.4 OA and OR kinetic models

The kinetics of crystal growth of nanostructures are dependent on the nature of the material, the type of crystal facet interface, the solvent, temperature, and concentration of the surfactant [28]. Both OR and OA crystal growth mechanisms can occur simultaneously, and their kinetic models were developed with respect to the diameter growth of a nanostructure. However, it is possible to change the crystal growth into an OA mechanism by hindering OR mechanism initially by introducing surfactants [28, 58]. It is very important to know the characteristics of the OR crystal growth mechanism to distinguish from the OA mechanism. As the OR mechanism is a diffusion-controlled process, it is favored thermodynamically with saturated solution that dissolves nanoparticles. In contrast, OA mechanism dominates in undersaturated conditions [59]. The crystal growth kinetic models of these OR and OA mechanisms also behave differently. OR mechanism-based kinetics models follow first-order kinetics while OA-based kinetic models follow second-order kinetics [60, 61].

The OR kinetic model was proposed by Lifshitz, Slyozov, and Wagner named as LSW kinetic model [2, 62]. The first-order equation for the linear crystal growth rate can be expressed by an exponential function as follows [57].

$$\left.{\mathrm{D}}^{\mathrm{n}}-{\mathrm{D}}_0^{\mathrm{n}}=\mathrm{k}(\mathrm{t}-{\mathrm{t}}_0\right)$$E2

where D and D₀ are the mean particle sizes at time t and t₀, k is a temperature-dependent rate constant, n is an exponent related to the coarsening mechanism through the diffusion.

The crystal growth kinetic process in OA is complicated than in OR kinetics since nanocrystals in the different stages of the reaction go through the collision and coalescence during the attachment. Therefore, three kinetic models have developed to explain the OA-based crystal growth of nanocrystals [2, 61]. They are: (1) primary particle-primary particle model (A₁+A₁), (2) primary particle-multilevel particle (A₁+A_i) model, and (3) multilevel particle-multilevel particle (A_i+A_j) model, developed by Smoluchowski based on the collision between number of particles [2]. Table 1 demonstrates the population growth matrixes of these three OA kinetic models. However, these modified Smoluchowski equations can describe the nanoparticle’s diameter growth of the reaction. The kinetic models that describe the elongational growth of 1D nanostructures are essential to explain the controlled fabrication of anisotropic nanostructures. Very few metal oxide/hydroxides anisotropic structures were explained by fitting the existing three kinetic models to understand the OA crystal growth mechanism, and they are not satisfactory enough to understand the growth rates.

Oriented-attachment-based kinetic models
A1+A1 model	A1+Ai model	Ai+Aj multistep model
$\mathrm{d}=\frac{{\mathrm{d}}_0\left(\sqrt[3]{2}{\mathrm{k}}_1\mathrm{t}+1\right)}{\left({\mathrm{k}}_1\mathrm{t}+1\right)}$	${\mathrm{d}}_{\mathrm{eq}}=\sum {\mathrm{N}}_{\mathrm{k}}{\mathrm{d}}_{\mathrm{k}}^4/\sum {\mathrm{N}}_{\mathrm{k}}{\mathrm{d}}_{\mathrm{k}}^3$	${\mathrm{d}}_{\mathrm{eq}}=\sum {\mathrm{N}}_{\mathrm{k}}{\mathrm{d}}_{\mathrm{k}}^4/\sum {\mathrm{N}}_{\mathrm{k}}{\mathrm{d}}_{\mathrm{k}}^3$
d and d0 are mean diameter at time t and t0	deq—average particle size	deq—average particle size
	dk—size of the secondary particle containing (1+i) primary particles	dk—size of the secondary particle containing k primary particles
k1 = rate constant	Nk—number of secondary particles	Nk—number of secondary particles
	Collision Matrix	k × k matrix
	$\left[\left(1+2\right)\left(1+3\right)\left(1+4\right)\left(1+5\right).\dots \left(1+\left(i-1\right)\right)\left(1+i\right)\right]$	$\left[\begin{array}{c}\left(1+1\right)\left(1+2\right)\left(1+3\right)\left(1+4\right)\dots \left(1+k\right)\\ \left(2+2\right)\left(2+3\right)\left(2+4\right)\dots \left(2+k\right)\\ \left(3+3\right)\left(3+4\right)\dots \left(3+k\right)\\ \left(i+i\right)\dots \left(i+k\right)\\ \left(\left(k-1\right)+\left(k-1\right)\right)\left(\left(k-1\right)+k\right)\\ \left(k+k\right)\end{array}\right]$

Table 1.

Three basic kinetic models OA-based crystal growth of nanoparticles [2].

2. Characterization techniques to visualize OA crystal growth dynamics

An in-depth understanding of the guiding principles of OA mechanism that dictates the attachment of adjacent nanocrystals toward a specific crystallographic orientation is still critical for the progress of miniaturization and high aspect ratio of anisotropic 1D nanostructures. The visualization tools including in-situ and ex-situ electron microscopy techniques and computational simulation methods have been investigated to reveal the OA crystal growth processes. It was found out multiple factors influence the OA process such as the type of the solvent or surfactant in the medium and their polarity, size and shape of the primary nanocrystal, temperature, and concentration of precursors [28, 58, 63]. The size and shape can be controlled by changing either surface energies of the crystal facets and the external growth environment [31]. However, the deeper understanding to obtain ultrathin 1D nanostructures is still in its infant stage. In this section, we discuss the main characterization tools that visualize the OA crystal growth process performed in different anisotropic nanomaterials and their findings.

2.1 Ex-situ investigation techniques

The ex-situ investigation is the technique that performs outside of the reaction process such as after the drying or processing. The disadvantage of this method is that artifacts induced to the system after the post process may affect the interpretation of the analysis. Therefore, it needs the careful analysis with other supporting techniques. Transmission electron microscopy (TEM) is an advanced, versatile, and standard ex-situ tool to obtain the structural and chemical information of the nanocrystalline materials such as elemental composition and mapping, size, shape, and crystallinity [64]. Selective area electron diffraction (SAED) feature associated with the TEM is used to determine the crystal structure and their crystallographic orientation in a specific area of the nanostructure [65]. This high accelerated electron beam related technique is enable to characterize the uniformity of nanomaterials [64]. The scanning transmission electron microscope (STEM) mode is one of the recent advancements of TEM, which generates images by performing a raster scan on the surfaces of nanostructures [64]. This STEM detector is coupled with a high-angle annular dark-field (HAADF) detector and the energy-dispersive X-ray detector (EDX or EDS) that provides the elemental composition and mapping in the nanostructures [66].

Today’s HR-TEM can provide atomic-resolution intrinsic structure, crystal lattice information combined with the chemical information of a single nanocrystal [67]. Therefore, it is one of the frontier-characterization tools that provide the understanding of the size and shape-controlled anisotropic nanostructures. One of the extensive studies of identifying the possible crystal growth mechanism of Ag nanowires was performed by Murph and her team utilizing both HR-TEM analysis and molecular dynamics (MD) simulations [9]. They have monitored the intermediate stages of the synthesis to visualize the coarsening process of similar crystallographic facets of neighboring nanocrystals. MD simulation results have suggested that the dipole-dipole attraction causes the preferential crystallographic attachment of hydroxide ions on the surfaces of nanocrystals to produce these ultra-long Ag nanowires.

HR-TEM provides crystal defect information such as twins, misorientation, tacking faults, and phase transformation, and it is an important visualization technique to identify OA crystal attachment process [64, 68]. The very early reports of Penn and Banfield demonstrated the crystal defects of TiO₂ nanocrystals including the edge, screw, and mixed dislocations by looking at the lattice fringe details of crystals using HR-TEM [19]. They referred such defects as imperfect oriented attachment, which can be expected in natural and experimental conditions.

Time-dependent XRD technique is another important characterization tool used to investigate crystal growth planes during the OA growth process. Although it is not a standalone technique to identify a crystal growth mechanism, it provides a platform to track dynamics of crystal planes growth at different stages of OA process combined with the selective area electron diffraction (SAED) of TEM [69]. Our group reported such valuable investigation to observe the gradual coalescence, reorientation of crystal facets during the OA-based sol-gel-derived process of ultrathin Cu(OH)₂ nanowires [44]. Figure 5 shows the time-dependent SAED patterns and powder XRD traces of Cu(OH)₂ nanoarrays and nanowires at the different stirring time intervals followed by aging in a base-catalyzed sol-gel chemical process.

Figure 5.

Time-dependent SAED patterns along with powder XRD traces of Cu(OH)₂ nanocrystals at different stirring and aging time. Reproduced from reference [44] with permission from the Royal Society of Chemistry.

2.2 In-situ investigation techniques

The in-situ investigation tools are prominent techniques to accurately investigate the crystal growth mechanisms as it can perform real-time monitoring in the reaction solution without any further modifications. The recent advances of liquid-phase TEM is one of the leading tools for the direct visualization of different nanostructures [47, 70, 71, 72, 73]. The high spatial and temporal resolution of the liquid-phase TEM allows to comprehensively understand the underlying growth mechanism at an atomic scale. It also provides the information about crystals orientation and crystal defect formation during the nanocrystal’s attachment [74]. As a result, our understanding of nanocrystal nucleation, growth, and their dynamics has accelerated. However, it requires a careful interpretation as it has the electron beam effect, substrate effect, and some synthesis procedures are complex and incompatible with liquid phase-TEM [75, 76]. Therefore, this tool has been limited to few synthetic process and materials although it facilitates real- time monitoring. However, these in-situ techniques alone cannot be utilized to confirm the growth mechanisms. They should undergo the ex-situ analysis as a supporting information to prove the direct observation analysis.

The direct observation of metal hydroxide/oxide growth is less as most common metal oxide synthesis methods are not compatible to observe under liquid-phase TEM. The important calculations of translational and angular speeds of iron oxyhydroxide nanoparticles were performed during the OA growth by Li and the team for the first time using liquid-cell TEM [47]. Furthermore, the Pb₃O₄ nanocrystals coalescence, and growth rates were determined by another group during the OA growth along the [002] crystal facet using liquid-cell TEM [77]. Another recent study demonstrated the fivefold twinned Au crystal domains formation using real-time HR-TEM imaging [78]. In-situ monitoring ZnO nanorod formation reached a milestone by providing new insights into the dynamics of OA [79]. The driving forces and torques for both aggregation and alignment were determined using the individual trajectories and attachment events of several ZnO nanoparticle pairs. The OA mechanism was confirmed using lattice fringes observations and its Fast Fourier transform (FFT) analyses. Investigation on oriented attachment of Au nanoparticles was performed by Zhu et al using direct observation of liquid TEM. In this ligand-controlled reaction, they observed that the overlapped ligands follow the rotation into {111} orientation. The calculated ligand binding energy on {111} crystal facets is lower than that of other crystal facets, which causes the preferential attachment of Au nanoparticles [14].

Small-angle X-ray scattering (SAXS) is another important in-situ tool to get qualitative and quantitative understanding of the OA mechanism at different stages of the process. It can accurately determine the shape and size of nanostructures [80]. Recently, the nucleation and growth of Au nanoparticles were successfully investigated to understand the mechanism and kinetics using SAXS technique in the solutions [81, 82, 83]. An early attempt of direct probing of TiO₂ nanorods in the reaction solution was performed by Tsao and the group at different temperatures for different reaction times [84]. The time-dependent temporal evolution of SAXS profiles revealed the spontaneous alignment of the quasi-spherical particles with the time from initially formed spherical particles. They confirmed that this observation combines with the HR-TEM analysis.

2.3 Computational simulations

The computational simulation techniques are a growing field to interpret experimental observations to reveal the detailed information of key growth mechanisms. The main simulation methods are molecular dynamics (MD), density functional theory (DFT), ab initio, and Monte Carlo (MC) calculations. These methods provide insight into the thermodynamics, kinetics, and driving forces of crystal growth mechanisms [85, 86]. Although computational simulations involved with faster timescales than experiments, they provide a valid approach to understand crystal growth dynamics with a good agreement with experimental results.

The current arguments of the driving force for OA mechanism are controversial. The main assumption of OA is the reduction of surface energy of nanocrystals that causes the thermodynamic driving factor for the spontaneous attachment of nearby nanocrystals [19, 87]. However, recently some research works have suggested that OA is driven by physical driving forces such as van der Waals interactions [88] and/or dipole-dipole interactions [89, 90]. Computational simulations are important to compare these different driving factors and then finally find out the primary contributing factor in different systems. Zhang and Banfield successfully analyzed these key factors separately by treating the system differently [86]. They demonstrated that Coulombic interactions are predominant when nanoparticles are close to each other in a solution, while van der Waals interactions are dominant when nanoparticles are far apart in a solution.

The main fortuitous advantage of combination of atomistic computer simulation techniques with experimental characterization methods is to validate the assumptions of crystal growth mechanism and controlling factors observed in the experiments. MD simulation is useful to explain the driving forces for the ex-situ TEM observations of different nanostructures. Zhang et al described the direction-specific interaction forces that can create torque to align adjacent ZnO nanocrystals and induce OA to form ZnO nanorods using MD simulations [91]. Very early studies of MD estimated the free energy changes of MgO nanoparticles due to the OA aggregation toward specific orientation in vacuum [92]. Another study used ab initio methods to calculate the surface energies of crystal faces of Sb-doped SnO₂ nanocrystals to predict the final morphology, which is observed from ex-situ TEM [93]. Murph et al also used MD studies to validate their HR-TEM experimental data on Ag nanowires and describe the mechanism to form penta-twinned nanowires growth along the [110] facet [9].

Very recent work demonstrated the solvent effect for the probability of perfect and imperfect OA mechanism by mapping the crystal growth dynamics of Ag nanocrystals in contrast to the solid state without the solvent in the medium [94]. This study is useful to develop new solvent directed strategies to control the specific crystallization processes to obtain the desired final product. Furthermore, Sayle and the group predicted the mechanical properties of CeO₂ nanorods and nanochains using atomistic computer simulation with the in detailed discussion of the effect of dislocations and grain boundaries for mechanical properties [95].

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 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 particle-particle 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 sol-gel chemical process [103]. By assuming that the sol-gel process is a quasi-homogeneous 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 zeroth-order 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 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.

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].

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 d-spacing of a nanocrystal confirms the Cu(OH)₂ crystal structure.

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.

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

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.

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

4.3 Lack of efficient and greener synthesis methods to make ultrathin anisotropic nanostructures

The controlled synthesis of ultrathin anisotropic nanostructures is limited to few materials, and the more efficient, scalable, greener, facile, and reproducible synthesis techniques are rarely introduced. The attempts of developing new strategies are needed to enhance the prospects for the preparation of controlled 1D nanostructures. Although there is a rapid progress on OA-based 1D metal/metal oxide nanostructures without any organic additives, we are still far away from the full understanding of OA crystal growth. The new efficient and greener synthesis methods will certainly open doorways toward rationally designing different ultrathin nanostructures. Although most common method to produce 1D noble metal nanostructures is seed-mediated growth methods, they are ineffective methods to make long nanostructures such as nanowires. Therefore, enough attempts are required to change the aspect ratio of 1D noble metal nanostructures with in-depth understanding of the controlling factors. The new solution-based wet chemical strategies are required to develop unrestrictedly size and shape-controlled 1D nanostructures to get the advantages of cost-effectiveness, energy effectiveness, and scalability of the process.

4.4 The insufficient theoretical investigation to improve fundamental understanding

The theoretical investigations further advance our fundamental understanding on OA crystal dynamic growth processes. Molecular dynamics, DFT simulations, molecular mechanics, ab initio methods are a few tools that can be used to develop theories and strengthen the understanding of the OA mechanism. The necessity of these theoretical tools combined with experimental techniques is increasing demand for further insight into the OA growth at different levels including molecular, atomic, and crystal lattice scale. If we combine real-time crystal growth dynamics with simulations of interparticle forces and interactions, it will provide the explanation of guiding factors quantitatively. However, the utilization of theoretical models to develop more accurate experimental OA growth models is limited, thus necessitating collaborative experimental work for further advancement.

5. Conclusions

In conclusion, the in-depth discussion of potential of “oriented attachment” mechanism has been overviewed in size and shape-controlled inorganic metal/metal oxide 1D nanostructures to use in different applications. The understanding of the interplay between OA crystal growth mechanism, phase transformation, and kinetics is the key to produce controlled 1D nanostructures, but the establishment of their relationship remains a tremendous challenge. The recent advances on in-situ, ex-situ techniques and theoretical simulations provide important insights into OA growth. However, the applicability of general kinetic models to interpret the OA mechanism was already questioned recently, as the growing interest of 1D nanostructures based on OA crystal growth. Therefore, the development of OA-based elongation kinetics of 1D nanostructures represents a key knowledge gap and a challenging task. Our recent time-dependent HR-TEM study provides new insights into crystal growth of Cu(OH)₂ nanowire formation from primary nanocrystals via OA mechanism in a sol-gel system. We introduced novel multistep sigmoidal kinetic models for the OA-based Cu(OH)₂ nanowire formation. This study significantly contributes to the advancement on the fabrication of ultrathin nanowires using OA attachment without using any surfactants by correlating OA-based elongation growth kinetics. We anticipate that the next decade will be an exciting time for both materials scientists and computational scientists to study crystal growth dynamics with state-of-the-art advanced instruments.

Acknowledgments

This work was performed at the Joint School of Nanoscience and Nanoengineering, a member of the Southeastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), supported by the NSF (grant ECCS-1542174). Financial support for this work was provided, in part, by the Joint School of Nanoscience and Nanoengineering, the University of North Carolina at Greensboro Office of Research and Sponsored Programs, National Science Foundation Science and Technologies for Phosphorus Sustainability (STEPS) (CBET-2019435). The authors acknowledge the DOD HBCU/MSI instrumentation award (contract #: W911NF1910522) to acquire HR-TEM (JEOL 2100PLUS) with STEM/EDS capability.

Conflict of interest

There is no conflict of interest to declare.