Advanced Supramolecular Nanoarchitectonics

By Katsuhiko Ariga

Advanced Supramolecular Nanoarchitectonics provides the latest information on design at the nanoscale, presenting a range of the new challenges that arise when the manipulation techniques that work at the macro- and micro-scale do not work at the nanoscale. Nanoarchitectonics approaches material design via a profound understanding of the interactions between individual nanostructures and their organization. This book presents the more advanced features of this research paradigm, focusing on the materials fabrication of self-defined nanostructures and demonstrating possible applications of these fabricated materials.

• Written by the team that coined the term nanoarchitectonics, providing a detailed explanation of the approach and techniques of supramolecular nanoarchitectonics
• Focuses on the fabrication of materials with defined structures via organized interactions at the nanoscale
• Demonstrates current and potential applications of nanoarchitected materials

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 23, 2019
• ISBN: 9780128133422

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China

Preface

Katsuhiko Ariga; Masakazu Aono

Innovation for new materials is always driven by necessity from thought. Activation of this thought requires materials and systems with high efficiency, that are compact and user-friendly and ecologically sound. In order to satisfy these requirements, scientists and engineers have to control the precise interior structure of nanostructures. The observation, characterization, fabrication, property evaluation, and application-purposed functionalization of these structures have to be undertaken with nanoscale precision. Nanotechnology plays an important role in these tasks, especially observation, characterization, and evaluation. Fabrication and functionalization using nanoscale precision requires further advances in nanoscience and nanotechnology.

However, fabrication on the nanoscale is not always easy. Phenomena and working principles at the nanoscale are highly influenced by thermal and statistical fluctuations, as well as mutual interactions between components. Therefore, harmonization of various factors is crucial when materials are architecturally combined using nanoscale building blocks. In addition to precise nanotechnology protocols, combining methodologies from neighboring research fields such as supramolecular self-assembly/self-organization becomes important. This unified concept, termed nanoarchitectonics (nano + architectonics), has been recently proposed.

The term nanoarchitectonics was first used in a conference title: First International Symposium on Nanoarchitectonics Using Suprainteractions in Tsukuba, by Masakazu Aono in Japan (2000), which is probably the first use of the combined term in the scientific community. The nanoarchitectonics concept notes that:

●Nanomaterials or nanosystems with functional reliable functions can be contructed by organizing nanoscale parts even with some unavoidable unreliability.

●Functions are not always originated only from the individual nanoparts but their interactions may create new functionalities.

●Unexpected functionalities can be emerged from assembling or organizing a huge number of nanoparts.

The first book in this series was called Supra-Materials Nanoarchitectonics. In this second book, Advanced Supra-Materials Nanoarchitectonics, various hot topics from basic fabrication strategies to advanced application-oriented examples are assembled and cover the areas of: (1) nanoarchitectonics of low-dimensional materials; (2) nanoarchitectonics of nanostructured materials; (3) nanoarchitectonics for applications; and (4) nanoarchitectonics with advanced materials. The reader will be able to see both the current frontiers and the future of materials science and technology involving nanoarchitectonics.

Section 1

Nanoarchitectonics of Low-Dimensional Materials

Chapter 1.1

Low-Dimensional Nanomaterials

Mekuriaw Assefa Kebede*; Toyoko Imae*,†    * Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taiwan, Republic of China

† Department of Chemical Engineering, National Taiwan University of Science and Technology, Taiwan, Republic of China

Abstract

Low-dimensional nanomaterials with a size of 1–100 nm exhibit the distinctive features responding to their specific characteristics. Their features depend on the synthesis routes (top-down or bottom-up procedures) and the growth methods in solid, liquid, vapor and hybrid phases. Products are finally classified into 0, 1, 2, or 3 dimensional materials based on their sizes in each dimension (x, y, or z) measured in nanoscale size range. They are representative by nanoparticles, nanorods, nanofilms, and nanocrystals. Therefore, in this chapter, the type, classification and synthesis of low-dimensional materials are briefly summarized, and their important roles towards the development of nanotechnology based on nanoarchitectonics are explained.

Keywords

Nanoarchitectonics; Low-dimensional material; Nanomaterial; Top-down/bottom-up procedure

1 Characteristics of Low-Dimensional Materials

Physical and chemical properties such as mechanical power, electric force, optical property, magnetism, thermal stability, coloration, and reactivity of a given bulk material are changed after the size of the material is down to a nanoscale level (1–100 nm) at least in one of its dimensions, and the resultant materials are called low-dimensional materials or nanomaterials (Fig. 1). Nanomaterials exhibit remarkable distinctive features such as the electronic wave functions of a confined state and the high surface-to-volume ratio compared to that of the bulk material. Thus, these features are responsible for the observed change of specific characteristics. For example, nanoparticles of a given material will have a lower melting point than that of the corresponding bulk material, because the nanomaterial contains a significant fraction of the total number of atoms or ions that influence the thermal stability [1]. For example, a noncatalytic gold ingot will exhibit excellent catalytic properties at its nanoscale size at room temperature [2].

Fig. 1 Dimensions of materials exhibiting nanometer-scale size.

Because the size and shape of nanomaterials determine the observed properties, different nanoarchitectonics are involved in the design and synthesis processes of nanomaterials to the desired size and shape. Consequently, there are many nanomaterials that have been produced and utilized in different applications of the science and technology arena.

2 Approaches to Syntheses of Low-Dimensional Materials

In general, two major approaches are used to prepare low-dimensional materials. They are top-down or bottom-up processes, as indicated in Fig. 2. The top-down approach involves the breaking down of bulk materials into nanomaterials. On the other hand, in the bottom-up synthesis method, either the precursors undergo the assembling or the chemical reaction towards the desired size and shape of nanomaterials. Then the precursors suffer thermal, chemical, or other appropriate treatments to synthesize the primary nanomaterials, which may further grow into the secondary nanomaterials. Both top-down and bottom-up techniques can be applied to the preparation of a given nanomaterial. For instance, bottom-up and top-down approaches have been used to synthesize monodispersed spherical colloids of metals such as Bi, Pb, In, Sn, and Cd, and their alloys [3]. Mono-dispersed bismuth particles in sizes of 100–600 nm can be synthesized either by a bottom-up approach through thermal decomposition of bismuth acetate in boiled ethylene glycol, or by a top-down approach after the emulsification of molten bismuth metal in boiling di(ethylene glycol).

Fig. 2 Different approaches of nanoarchitectonics: (A) bottom-up approach, (B) top-down approach.

The top-down approach is advantageous for the mass production of materials, but the products are rather large in size and inhomogeneous in size distribution, as the approach is mainly based on a mechanical fashion. Thus, to produce small and homogeneous nanomaterials, the bottom-up approach is preferred. For example, the polymerization process of monomers to polymer molecules and the growth of particles through the self-assembly of atoms, ions, or molecules are typical bottom-up approaches.

On the bottom-up approach, the wet chemical methods are often utilized, and their tunability is achieved by changing the preparation parameters such as type and concentration of the precursors, and pH, temperature, and pressure of the reaction systems. Many nanoparticles of different sizes and shapes such as silver (AgNp) [4], platinum (PtNp) [4], gold (AuNp) [5], zinc oxide (ZnO-Np) [6], ferric oxide (Fe2O3-Np) [7], and gadolinium (Gd2O3-Np) nanoparticles [8] have been prepared by using this technique. The characteristic behaviors of nanoparticles mainly arise from their high surface-area-to-volume-ratio and/or their quantum effect, which governs the overall electrical, optical, thermal, and catalytic properties [6, 9, 10]. Thus, the size and shape of nanoparticles can be controlled by means of nanoarchitectonics to obtain the desired functionalities.

The preparation techniques can be categorized by the growth in solid phase, liquid phase, vapor phase, and hybrid phase depending on the media where the particles are going to be fabricated: The atomic layer deposition of nanoparticles belongs to the vapor phase synthesis technique, whereas the colloidal processing for the formation of nanoparticles and the self-assembly of monolayers are examples of nanomaterial preparation technologies in a liquid medium. The combination of different growth mediums in hybrid phase techniques includes the vapor-liquid-solid growth of nanoparticles such as nanowires [11].

3 Classification of Low-Dimensional Materials

Nanomaterials can be classified based on shape such as nanoparticles, nanocrystals, nanowires, nanorods, nanospheres, nanoprisms, nanorings, and thin films. Moreover, they are also categorized into three classes based on the size of x, y, and z dimensions (see Fig. 1). In zero-dimensional (0D) nanomaterials, all three dimensions have nanoscale size and are appear as a dot. Nanoparticles, quantum dots, and nanospheres belong to this group. One-dimensional (1D) nanomaterials grow in one direction and have a diameter in a nanoscale size, such as nanowires, nanorods, and nanotubes. Two-dimensional (2D) nanomaterials expand in both x and y directions, whereas the third dimension remains in nano-meter size in case of thin films and layers. In 3D nanomaterials, all the three directions grow but maintain a nanoscale size and some lower-dimensional nanomaterials will be ordered as building blocks of 3D nanomaterials and result in the desired 3D structure. Multi-nanolayers, bundles of nanorods or nanowires, or ordered aggregates of nanoparticles (nanocrystals) with nanoscale features are among 3D nanomaterials. Fig. 3 shows representative structures of nanomaterials classified based on their dimensions.

Fig. 3 Different shapes of low-dimensional materials.

The characteristics of nanomaterials may change depending on the dimensions. The changes of optical and electronic properties occurring in the low-dimensional materials are related to band gap energy. As the size of the system becomes comparable to the de Broglie wavelength of the electron, the discrete nature of the energy states occurs [12].

4 Zero-Dimensional Nanomaterials

The difficulty with creating 0D nanomaterials is their easy agglomeration in solution, and even in powder form, because of the strong van der Waals attraction between nanomaterials. Thus, in most cases, a protection treatment is adopted during the synthesis or after producing 0D nanomaterials. Metallic nanoparticles can be prepared by the bottom-up approach through the reduction process in a wet condition by using chemical reducing agents or light irradiation (photoreduction). Silver nitrate is dissolved in water to produce silver ions through the ionic dissociation. Sodium borohydride is then added as a chemical reducer to convert the ions into the corresponding metallic silver nanoparticles [4]. In this case, although surfactants and microemulsions are typically utilized as stabilizers of nanoparticles and templates [13], a poly(amido amine) dendrimer molecule has been also used to capture silver ions inside its cavity by forming a metal ion-dendrimer complex before the subsequent reduction step [14]. This situation helps to retain the small size (less than 5 nm) of the resultant silver nanoparticles, while the dendrimer molecule also plays a significant role as a stabilizer to produce well-dispersed silver nanoparticles. Photoreduction is also available to convert metallic ions such as silver and gold ions to their metallic atoms [15, 16]. Metallic ions that are immobilized on the surface of photosensitizer such as carbon dots on a graphene sheet as a substrate are reduced to metallic nanoparticles [16]. The light is used to generate electrons from the sensitizer and the photogenerated electrons are transferred to the metal ions so that the corresponding reduced metallic nanoparticles can be obtained. The strong interaction between the functionalized substrate surface and the metallic nanoparticles is vital to stabilize such particles and enables them to be used for other purposes. For instance, gold nanoparticles prepared by this process can be applied for photoreduction of CO2 gas and then converted into a formic acid molecule because of electron concentrating and transferring properties of the prepared gold nanoparticles from the electron source to the CO2 molecule in an aqueous medium [16]. Gold nanoparticles can be deposited on the substrate through the thermal vapor evaporation in a vacuum chamber [17]. The as-prepared gold islands are applied for sensor use based on the characteristics of the surface-enhanced Raman scattering/infrared absorption/fluorescence spectra [18, 19].

The shape and size of metal oxides developed by the hydrolysis of metal ions on the bottom-up approach are highly dependent upon the preparation procedures, but metal oxides prepared by the polyol solvent method [6, 20] or the dendrimer-passivated method [21] are smaller than 10 nm and hence can be classified as 0D nanomaterials. Small sizes of ZnO nanoparticles will be more effective for photovoltaic electrochemistry [6] and dendrimer-protecting TiO2 nanoparticles are water dispersible and highly efficient for photocatalysis [21].

For the synthesis process of metal oxide nanoparticles, the embedding of materials within them increases the functionality of the nanoparticles. Silica-based ceramic nanoparticles have synthesized by the sol-gel method and, on the preparation process, the photosensitizer drug, 2-devinyl-2-(1-hexyloxyethyl) pyropheophorbide, has been encapsulated inside spherical SiO2 nanoparticles having a diameter of less than 30 nm, which are highly monodispersed and stable in an aqueous medium [5]. The system has shown good performance in significantly killing a tumor cell after the light radiation on cells that are taken up in the system. Fig. 4 shows some of 0D nanomaterials reported.

Fig. 4 Transmission electron microscopic images of (A) Pt, (B) Ag, (C) ZnO [4 , 6] , and (D) Fe nanoparticles [22]. (E) Scanning electron microscopic image of Au nanoparticles [5]. (F) Atomic force microscopic image of carbon dots [16]. Images reproduced with permission; (A), (B) and (C) from Elsevier, (D) and (E) from American Chemical Society and (F) Royal Society of Chemistry.

5 One-Dimensional Nanomaterials

Nanomaterials with high aspect ratio are considered 1D nanomaterials. They have nanometer scale sizes in both x and y dimensions. These include nanorods, nanotubes, nanowires, and nanofibers. Fig. 5 illustrates their morphologies. They are synthesized through different procedures, differently characterized, and display diverged potential applications.

Fig. 5 Scanning electron microscopic image of (A) ZnO nanorods [23] and (B) Multiwalled carbon nanotubes [24]. (C) Atomic force microscopic image of Au nanowires [25]. Images reproduced with permission from Elsevier.

Zinc oxide is an inorganic semiconductor with a band gap of 3.37 eV at room temperature. It shows advantages over other similar oxides, e.g., TiO2, with its low cost, good electron transfer, and low corrosion. Various technologies were reported regarding the synthesis of ZnO towards different morphologies such as nanorods [23, 26–28], nanowires [29–31] and nanofilms [32–34] as well as the nanoparticles described above. ZnO nanowires have received more attention from nanomaterial researchers because of their characteristic physical properties such as very large surface-to-volume ratio, high crystallinity, and simple preparation method [35]. Hence, they are appreciated in various nanotechnology areas such as solar cells [36], optoelectronics [37], and sensors [38, 39]. In medical applications, nanowires of ZnO (50–110 nm length) and silver have been knitted together to become a sheet that will serve as a glucose sensor to monitor glucose in blood [35]. The fabricated material has shown better detection sensitivity at a short response time.

Zinc oxide nanowires can be synthesized in a gas phase or liquid mediums. In a vapor-phase synthesis method, the vapor obtained from the mixture of ZnO and graphite powder after being heated to around 900°C under inert gas atmosphere for about 30 min results in ZnO nanowires grown on Au-coated silicon substrates [40]. Thus, the vapor-phase deposition synthesis provides nanowires with high quality and homogenous structure, although this technique usually requires a high reaction temperature. Hence, alternative low-temperature liquid-phase growth methods such as hydrothermal or electrochemical growth are also employed. These wet syntheses are more applicable because of the low cost, the various choice of precursor chemicals, the environmentally-friendly processes, and the possibility of large-scale production of nanowires.

In a hydrothermal procedure, ZnO nanowires are prepared from the reaction solution of zinc nitrate salts and hexamethylenetetramine after heating the solution to 90°C inside an autoclave [29]. The length and width of the wires depend on the reaction time and the concentration of zinc precursor. Moreover, the growth of nanowires on the substrate is evaluated by pre-depositing ZnO seeds on the given substrate. Although, as a result, highly densely arrayed ZnO nanowires can be obtained, the seeding creates defects of the nanowire array on the substrate due to the weak interaction between wires and substrate, the low thermal and electrical transports, and the optical properties.

The electrochemical growth of ZnO nanowires is a possible solution to decrease the defect-forming effect on the seed method and enables the synthesis of well-aligned nanowires with the desired properties [30].

Likewise, silver nanowires are also one of 1D nanomaterials getting receiving great attention because of their potential advantages in the fabrication of various electronic devices [25, 41–43]. The nanowire morphology is obtained by using different polymer stabilizers during the reduction process of silver ion to metallic silver that facilitates the accumulation and the growth of reduced silver atoms in one direction to obtain nanowires. For the synthesis of silver nanowires by the polyol method, silver ions are heated in an ethylene glycol solution of FeCl3 and poly(vinylpyrrolidone), resulting in polymer-insulated silver nanowires with a diameter of about 70 nm and a length of about 8 μm with one-directional growth and stabilized by polymers [44]. Although the reason for adding chloride salts such as FeCl3, NaCl, and ZnCl is not clarified, the presence of chloride ions played an important role the formation of Ag nanowires; however, spherical silver particles were the major products in the absence of such ions [45]. The prepared polymer-insulated silver nanowires can be used to improve the electrical conductivity of an electrode [44].

Apart from the metal- or metal oxide-based 1D nanomaterials described above, another type of 1D nanomaterials are linear polymers. In recent years, cellulose nanofibers have received much attention, because they are one of renewable materials prepared from pulp through top-down techniques, different from biopolymers and bottom-up synthesized polymers, and expected to have various applications in the current science and technology fields [4, 46–53]. Currently, the extraction of cellulose molecules from pulp is subjected to chemical treatments such as acid hydrolysis (defibrillation) by sulfuric acid or nitric acid and oxidation reaction by 2,2,6,6-tetramethyl-piperidinyl-1-oxyl (TEMPO) radicals. Thus, cellulose nanofibers (2–4 nm in width and several micrometers in length) can possess enough repulsive forces between fibers to be effectively dispersed in water by weakened hydrogen bonds [54–58]. Meanwhile, the mechanical disintegration of pulp by homogenizing grinders or high-power ultrasonic treatments produces microfibrillated cellulose by cutting the main chain of the cellulose microfibril structure [57].

Because TEMPO-oxidized cellulose nanofibers have viscoelasticity owing to a network originating in the hydrogen-bonding interaction involving carboxylic acid in aqueous medium, they may serve as a matrix to embed some functional molecules/materials, and the composites/hybrids behave as functional materials [59]. The strong luminescence under UV light from nanocomposites of TEMPO-cellulose nanofibers with luminescent quantum dots is a valuable property for the application of the nanocomposites in the fields of cellular imaging, tumor targeting, fluorescence probes, molecular imprinting, photovoltaic devices, light-emitting devices, and UV-protective materials [60]. Among others, cellulose nanofiber modified with the fluorescent organic compound of fluorescein isothiocyanate is applicable for fluorescence bioassay and bioimaging technologies [61]. The TEMPO-oxidized cellulose nanofibers become transparent films, even if they embed/graft the metal catalyst-passivating dendrimer and the films are water-insoluble [62, 63]. Thus, these composite films are reusable catalytic reactors in aqueous solution, antibacterial protectors, and gas (formaldehyde) captures/decomposers. The clay-embedded composites of TEMPO-cellulose nanofiber have been designed for CO2 and NH3 gas capture [64], and the photosensitizer/dendrimer-grafted and enzyme-loaded composite converts CO2 to methanol by the photo-induced enzymatic reaction [65].

6 Two-Dimensional Nanomaterials

The 2D nanomaterials exhibiting a nanometer scale size in one of their dimensions are nanofilms. On the top-down method, nano-sheets can be obtained by peeling off a multilayer precursor into pieces of separate sheets with the nano-scaled size such as graphene sheet from graphite. Meanwhile, in the bottom-up procedure, nanofilms can be prepared by 2D deposition or array of molecules or nanomaterials by techniques such as spin coating [66, 67], sputtering [68], dipping [69, 70], roll coating [71], electroplating, or electrochemical deposition [72–74]. The applicability of these procedures depends on the type of precursors (molecule or nanomaterial), substrate, and/or fabrication conditions of the desired nanofilm. Currently, nanofilms are utilized in the development of semiconductor films for electronics [67, 75, 76], photovoltaics [77, 78], displays [79], sensors [80], and insulating films [81].

A nanofilm of In2O3 of 20 nm width has been obtained through the atomic layer position technique, where the necessary precursors (cyclopentadienyl indium and an aqueous hydrogen peroxide solution) are sequentially injected into the reaction chamber, an inert (nitrogen) gas is purged before the addition of the next precursor, and the deposition is made by evaporating the precursors at 160–200°C [82]. The optical band gap, the surface roughness, and the atomic ratio between indium and oxygen of the obtained nanofilms change with an increase of temperature. These In2O3 thin films prepared by using different deposition techniques are applicable for various technological devices such as solar cell electrodes [83] and electronic displays [84] because of the optical transparent properties of the oxide film.

A metal oxide HfO2 nanofilm on hydroxylated silicon wafer has been prepared through the sol-gel procedure from hafnium(IV) n-butoxide precursor and an organic solvent mixture [85]. The organogels on the wafer are hydrolyzed and the chemisorption/hydrolysis procedure is repeated through 10 cycles to produce a HfO2 layer of around 6 nm thickness. The HfO2 nanofilm from this method is comparable to that from the vapor-deposition method when assessing the electrical properties, dielectric constants and leakage currents, but superior to that from the spin-coating method for the homogeneity of the layer.

A seed-mediated hydrothermal synthesis and a spin-coating procedure can be used to prepare a nanofilm of BiVO4 decorated with PbS quantum dots [86]. The seed layer of BiVO4 is first coated on a conductive glass substrate based by the solution combustion synthesis method. Next, the BiVO4 nanofilm is grown by a seed-mediated hydrothermal method on the seed layer of BiVO4. Finally, the dispersion of PbS quantum dots is twice spin-coated on the as-prepared BiOV4 nanofilm to result in the uniform deposition of PbS. For the application of the fabricated nanofilm as a solar cell photoelectrode, the incident photon-to-current conversion efficiency is ∼ 5.9% on excitation at 370–450 nm, which is almost double of the BiVO4 nanofilm photoelectrode without PbS.

Standalone composite films ranging in thickness up to the micrometer size can be prepared by embedding a (nanomaterial) filler in a reinforcing polymer matrix. The polymer matrix provides the extra stability for fillers or adds their specific functionality to result in a durable nanocomposite. Here, the amount and size of the nanomaterials, matrix, drying conditions, and/or pore size can be used to control the thickness of the film. Composite films of ZnO and TEMPO-oxidized cellulose nanofibers have been prepared at different proportions (0%–50%) of two components [87]. The size of the film becomes larger when the amount of nanoparticles used increases: The thickness of the composite film that contains only 10% ZnO nanoparticles is 10 μm but exhibits light transmittance more than 80% at a 600 nm wavelength. Another self-standing nanocomposite film is fabricated by using bacterial cellulose (BC) as a matrix to stabilize graphene and titanium dioxide (GO-TiO2) together, and the film showed excellent photodynamic antibacterial activity due to its reactive oxygen generation property. Fig. 6 shows representative images of nanofilms prepared on substrates (A) and self-supporting films (B and