Anisotropic and Shape-Selective Nanomaterials

Globalization of scientific knowledge and technological advances are sparking innovation and creativity across many fields at an unprecedented rate. Ground-breaking discoveries made in the mid-1980s, namely the development of scanning tunneling microscopy and the discovery of buckminsterfullerene, influenced scientists to envision the world at the atomic level and new paradigms emerged: nanoscience and nanotechnology. Through the manipulation of matter at the atomic level, today scientists can create novel materials with unique properties and functionalities. These new materials enable innovative technologies and applications across many fields from engineering to medicine.

As material size decreases into the nano size regime, novel properties arise that are different from their molecular and bulk counterparts. Due to the size and shape effects in this regime, a nanoparticle’s morphology has a profound effect on its properties. This chapter addresses the effect of dimensionality on the optical, electronic, chemical, and physical assets of various nanomaterials and how physical and chemical relationships can be exploited to improve their properties. Delving into the nuances of the different sizes, shapes, and compositions gives one an appreciation of the potential that nanomaterials have to improve upon today’s technologies. As scientists learn to fabricate increasingly more complex nanomaterials, new opportunities develop every day. A detailed discussion on the effect of morphology and nanometric dimensions on materials' physico-chemical properties, which lead to novel applications, will be covered in Chapter 5.

This chapter gives an overview of the various approaches that have been taken to create anisotropic nanomaterials. The synthetic mechanisms of nanomaterials are being actively pursued due to the unique size and shape dependent properties that can be exploited for a myriad of applications. Nanomaterials have been synthesized in a gamut of shapes, sizes, and compositions. As their synthetic protocol progresses, scientists are becoming more and more creative in the fabrication of nanomaterials with very interesting architectures to tailor their properties for faster electronics, better resolution imaging, more efficient catalysts, among others.

Research into shape-selective and anisotropic nanoparticles is generally motivated by the desire to create better materials for a specific application, and therefore, it is critical to understand how and why shape affects nanoscale properties. Such information can be revealed through analytical experimentation, and this chapter describes characterization methods and challenges associated with analyzing anisotropic and shape-selective nanoparticles. Researchers can typically employ commonly available techniques used in materials characterization. However, in the case of anisotropic and shape-selective nanoparticles, greater concern for orientational and/or shape effects and artifacts should be shown during analyses.

In this chapter, we highlight recent innovations from our laboratory by featuring uniquely shaped nanostructures and how their morphology and dimension affect their physico-chemical properties and subsequently their applications. We aim to cover a wide range of applications including optical and plasmonic applications, sensing and imaging, catalytic and photocatalytic applications, bio-medical and environmental implications as well as energy related applications.

The development of nanomotors (nano- and micron sized particles that convert energy into mechanical movement) is an exciting endeavor. Nanomotors have been crafted in an extensive variety of sizes, morphologies and compositions for applications such as drug delivery, cargo transport, sensing, and lithography. Inspired by nature’s elegant use of chemical gradients and cellular tracks for independently driven molecular processes, a variety of machines have been created. With the recent bestowment of the Nobel Prize for molecular machines, this concept is being actively pursued to create inorganic nano- and microparticles that independently move for a gamut of applications.

This chapter describes the use of colloidal successive ionic layer addition as an additive growth method to form inorganic colloidal nanocrystals with controlled shapes. Rational design of nanocrystal dimensions and layer thicknesses in nanocrystal heterostructures is important to many current and anticipated uses of nanocrystals in optoelectronics, energy conversion, and fluorescence imaging. One approach to shape control is the use of a series of self-limiting surface reactions to build up crystals one atomic layer at a time. This approach is especially applicable to nanocrystals made of binary ionic or polar-covalent crystalline compounds, as are found in colloidal quantum dots (QDs). The intrinsic symmetry present in colloidal nanocrystal nuclei can be suppressed to enforce conformal layer growth or harnessed to promote regioselective growth. We specifically discuss two families of methods that are colloidal analogues of vapor-source atomic layer deposition. In colloidal successive ionic layer adsorption and reaction (colloidal SILAR), reagents are introduced to a nanocrystal solution in metered doses corresponding to the total surface area. In colloidal atomic layer deposition (colloidal ALD), reagents are added in excess, and unreacted reagent is subsequently separated and removed. An extensive literature exists on the use of colloidal SILAR to form nominally isotropic core/shell quantum dots (QDs) with high photoluminescence quantum yield. Colloidal ALD has been introduced more recently. There is increasing interest in applying both methods to the formation of anisotropic nanocrystal heterostructures, both through deposition of conformal layers on anisotropic substrates and through controlled anisotropic growth. We review the historical development of these methods, common precursors, and recent developments in monitoring of reaction progress and mechanisms. We also present contemporary examples of isotropic and anisotropic growth, and prospects for future development in the context of several representative applications including cell membrane voltage measurements and fluorescence anisotropy.

The plasmon drag effect was discovered about a decade ago in 2005. Since then, it has attracted considerable attention from the nanotechnology and photonics community due to fundamental physics of this effect as manifestation of electron plasmon coupling and myriad of possible applications in nanoelectronics, photonics and sensing. In this chapter, we review the recent advances in the plasmon drag effect studies.

Hybridization of nano-scale entities lead to higher dimensional ensemble materials with multifunctionality. Such hierarchical complex materials though are engineered with output properties in mind, these evolved nanostructures possess unique shapes and physico-chemical attributes. Nanotoxicological considerations hinge on physical size and shape factors; thus, dramatic alterations to shape and dimensionality of ensemble nanohybrids (NHs) necessitate careful evaluation of this ‘horizon’ material class. This chapter reviews size/shape/dimensionality variations of nanomaterials due to hybridization and discusses property alteration of these NHs, relevant to applications and nanotoxicology. The chapter also discusses nano-bio interactions of novel nanohybrids in relation to their size, shape, and dimensionality, and outlines future research needs and strategies.

Inorganic colloidal nanoparticles (NPs) possess the ability to self-assemble into complex hierarchical structures with unique properties that are different from their individual counterparts. The assembly of particles into compact 2D and 3D structures occurs only when they have a narrow size distribution and uniform shape. In this chapter, we review how various types of forces and fundamental interactions at the nanoscale govern the assembly of colloidal nanocrystals, the available methods for assembling colloidal nanocrystals with discrete geometries and the application of such assembled structures in various fields ranging from catalysis, biological diagnosis, plasmonics and electronics.

The electrochemical conversion of CO₂ into carbon-based fuels has attracted considerable attention as a promising strategy for closing the anthropogenic carbon cycle. A key challenge for achieving this goal is to develop selective, stable, and efficient electrocatalysts for the electrocatalytic reduction of CO₂. Nanostructured catalysts can provide many advantages compared to bulk materials, including the increase of active sites, the change of the local pH of the electrolyte, and improved stability. This chapter reviews the recent development of nanostructured metal catalysts for the electrocatalytic reduction of CO₂, mainly focusing on the fabrication, characterization, catalytic performance, and the reaction mechanism of these materials. In addition, the recent utilization of nanostructured bimetallic catalysts are introduced and a fundamental understanding of the reaction mechanism for their ability to reduce CO₂ is discussed. Finally, nanostructured carbon is shortly reviewed due to its low cost and improved catalytic activity and stability for the electroreduction of CO₂.

The design and synthesis of well-defined nanoscale anisotropic particles is opening new avenues toward developing a fundamental understanding of their chemical and physical properties. Obtaining nanoparticles that are homogenous in size and shape can be a challenging process, particularly for particles that consist of reactive metals. Anisotropy further adds to the complexity in controlling the kinetics and thermodynamics of the nucleation and growth processes. Access to well-defined nanoparticles provides the ability to develop succinct pathways toward elucidating reaction mechanisms they mediate as well as understanding their stability in various environments. Thus, significant effort in the field of nanoscale science and technology has focused on developing procedures that are reproducible in yielding well-defined nanoparticles. This chapter reviews various methods for the synthesis of catalytic metal nanoparticles and the impact of their shape on the reactivity. Methodology for the characterization of the nanostructures is also described.

Synthesized colloidal suspensions of gold nanoparticles (GNP) have been around since the early 1950s. Since that time there has been an extremely large body of work dedicated to investigating both the chemical and physical properties of these fascinating materials. This work has by no means been limited to nanospheres, but over the last ~20 years included a wide variety of morphologies including nanorods, nanocubes, nanoshells, and nanocages to name a few. There has been much effort spent on functionalizing these nanomaterials for the purpose of novel nanotechnology-driven approaches to medicinal and biological challenges. Gold nanoparticles continue to be of interest due to their interesting optical properties, sizes, photothermal properties and long-term stability. More recently, there has been a more concerted effort to better understand the growth mechanism of the gold nanoparticles and to discover more efficient and/or greener alternatives for the synthesis of these materials. We cannot begin to provide a truly comprehensive explanation of all of the published work within the gold nanoparticle field in this chapter and so have chosen to outline a few key areas of study.

Cardiovascular disease is a significant global Cardiovascular disease is a significant global health problem. Effectively treating it and exercise, but also demands the development of novel tools for rapid diagnosis and new therapeutics for treatment. The field of nanomaterials is making significant contributions to multiple health care problems in the areas of disease detection, imaging and drug delivery. Gold nanoparticles are particularly promising due to their ease of synthesis, biocompatibility and unique optical properties. In particular, gold nanorods having received much attention for their potential in the diagnosis and treatment of cancer, are now being examined for other biomedical applications. This chapter highlights efforts using gold nanorods in cardiovascular research in such areas as detection of cardiovascular disease, understanding cardiac cell response to nanomaterials and the ability of gold nanorods to alter the mechanical properties of model tissue constructs and cardiac valves.

This paper has reviewed the frontier field of “architectured membranes” that contain anisotropic oriented porous nanostructures of inorganic materials. Three example types of architectured membranes were discussed with some relevant results from our own research: (1) anodized thin-layer titania membranes on porous anodized aluminum oxide (AAO) substrates of different pore sizes, (2) porous glass membranes on alumina substrate, and (3) guest-host membranes based on infiltration of yttrium-stabilized zirconia inside the pore channels of AAO matrices.