Comprehensive Guide to Nanochemistry and Nanomaterials Classification

Nanomaterials Classification and Dimensionality

• General Classification Criteria: The classification of nanomaterials is primarily determined by the number of dimensions that are not confined to the nanoscale range, where the nanoscale range is defined as strictly less than $100\,nm$.
• The Four Dimensional Classes:     - Zero-dimensional (0-D): All dimensions are within the nanoscale range ($x, y, z < 100\,nm$).     - One-dimensional (1-D): Two dimensions are within the nanoscale range ($x, y < 100\,nm$), while the third dimension ($L$) is not confined and remains at the macroscale ($> 100\,nm$).     - Two-dimensional (2-D): Only one dimension is within the nanoscale range ($d < 100\,nm$), while the other two dimensions ($L_x, L_y$) are at the macroscale ($> 100\,nm$).     - Three-dimensional (3-D): No bulk dimension is confined to the nanoscale (all > 100\,nm); however, these materials possess nanocrystalline structures or features at the nanoscale.

Zero-Dimensional (0-D) Nanomaterials

• Definition: Materials where all measurable dimensions are within the nanoscale range ($\le 100\,nm$ in all directions).
• Common Representations:     - Nano spheres: Spherical particles where the diameter is $\le 100\,nm$.     - Nano cubes: Cubic structures with side lengths $\le 100\,nm$.
• Specific Examples:     - Cu Nanoparticles: These are described as being almost spherical in shape, as evidenced by Transmission Electron Microscopy (TEM) imaging.     - Quantum Dots: Closely packed semiconductor crystals composed of hundreds to thousands of atoms.     - Fullerenes: Spherical carbon molecules such as $C_{60}$.     - Clusters and Atomic Aggregates: Small groupings of atoms with dimensions in xyz < 100\,nm.

One-Dimensional (1-D) Nanomaterials

• Definition: Materials that have one dimension measuring outside the nanoscale range (> 100\,nm), while two dimensions are confined to the nanoscale ($\le 100\,nm$).
• Structural Varieties:     - Nanotubes: Hollow cylindrical structures.     - Nanorods: Solid rod-like structures.     - Nanowires: Thin wire-like structures.
• Specific Examples:     - Oxide Nanorods: Visualized through TEM images showing consistent diameter under $100\,nm$ but length exceeding $100\,nm$.     - Carbon Nanotubes (CNTs): Cylinders consisting of one or more layers of graphene.     - Nanobars and Nanoribbons.

Two-Dimensional (2-D) Nanomaterials

• Definition: Materials where two dimensions are not confined to the nanoscale (> 100\,nm), resulting in plate-like or sheet-like shapes. Only the thickness is confined to $\le 100\,nm$.
• Common Forms:     - Nano-films.     - Nano-layers.     - Nano-coatings (applied to bulk materials).
• Specific Examples:     - Graphene: A single layer of carbon atoms.     - Graphene Oxide.     - Two-layered Graphene.

Three-Dimensional (3-D) Nanomaterials

• Definition: Materials not confined to the nanoscale in any bulk dimension (all dimensions are arbitrarily above $100\,nm$), but which involve nanoscale internal features.
• Structural Characteristics:     - Nanocrystalline Structure: Bulk materials composed of multiple arrangements of nanosize crystals, typically in different orientations.     - Feature Dispersions: 3-D volumes containing dispersions of nanoparticles, bundles of nanowires, bundles of nanotubes, or multi-nanolayers.
• Specific Examples:     - Graphite: The macroscale bulk form of carbon layers.     - Polycrystals.     - Diamond.     - Metal-Organic Frameworks (MOF).     - Pillared Graphene and Aerogels.     - Nanocomposites: Materials that combine different nanoparticles or nanoparticles with bulk materials to achieve synergistic properties.

Classification by Material Composition

• Nanomaterials are further organized into four distinct types based on their chemical makeup:     1. Carbon based materials.     2. Metal based materials.     3. Dendrimers.     4. Composites.

Carbon Based Materials: Overview and Forms

• General Description: These materials are composed primarily of carbon and take the form of hollow spheres, ellipsoids, or tubes.     - Fullerenes: Spherical and ellipsoidal forms.     - Nanotubes: Cylindrical forms.
• Significance of Carbon:     - Basic element of life.     - Ability to bond to many elements in diverse ways.     - The sixth most abundant element in the universe.
• Principal Allotropes:     - Diamond.     - Graphite.     - Fullerenes.     - Carbon Nanotubes.

Graphite and Graphene

Graphite

• Stability: Regarded as the most thermodynamically stable form of carbon.
• Conductivity: High electrical conductivity, making it ideal for batteries, electrodes, and solar panels.
• Structure: Consists of layers or sheets of graphene stacked together.
• Lattice Constants:     - Hexagonal lattice carbon atom separation: $0.142\,nm$.     - Distance between planes: $0.335\,nm$.     - $C_0 = 0.671\,nm$.
• Industrial Applications: Heating elements in non-oxidizing atmospheres, metallurgical crucibles, casting molds, electrical contacts, brushes, resistors, high-temperature refractories, welding electrodes, and air purification systems.

Graphene

• Discovery: First isolated by A.K. Geim and K.S. Novoselov at the University of Manchester in 2004. They received the Nobel Prize in 2010.
• Molecular Structure: A 2D crystalline allotrope with a hexagonal pattern. Each carbon atom forms four bonds: three $\sigma$ bonds ($sp^2$ hybridized) with neighbors and one $p$ bond oriented out of plane.
• "Mother of All Carbon Nanomaterials": It is the basic structural unit for graphite, fullerenes, nanotubes, and nanocones.
• Properties:     - Nearly transparent.     - 200 times stronger than steel by weight due to tightly packed atoms.     - Highly efficient heat and electricity conductor due to $p$ electrons.
• Modern Uses: Semiconductors, batteries, electronics, and composite industries.

Diamond Structure and Properties

• Structure: A solid form of carbon with atoms arranged in a "diamond cubic" crystal structure.
• Bonding: Purely covalent chemical bonding with a highly symmetrical unit cell.
• Physical Properties:     - Extremely hard.     - Low electrical conductivity.     - Superior high thermal conductivity.     - Optically transparent.
• Primary Uses: Gemstones, industrial grinding, machining, and cutting tools.

Fullerenes

• Structural Composition: Graphene sheets rolled into spheres or tubes. The most famous is the Fullerene ($C_{60}$), often called a Buckminster fullerene.
• Molecular Design: A cage-like structure of 60 carbon atoms joined by single and double bonds. It consists of 20 hexagonal faces and 12 pentagonal faces, resembling a football.
• Physical state: A black solid with free-moving electrons that allow for electrical conductivity.
• Medical and Practical Applications:     - Antiviral Activity: Potential use in HIV-infection treatment.     - Targeted Drug Delivery: Can bind specific antibiotics and target cancer cells (e.g., melanoma).     - Biological Antioxidants.     - Photodynamic Therapy: Used as photosensitizers.     - Catalysis: Hydrogenation catalysts.     - Lubrication: When incorporated with sulphides of tungsten and molybdenum, they show excellent solid-lubricant properties.     - Molecular Caging: Can cage other molecules for slow-release drug delivery.

Carbon Nanotubes (CNTs)

• Discovery: Identified in 1991 by Iijima Sumio of Japan.
• Dimensions: Tube-shaped materials with diameters ranging from less than $1\,nm$ to $50\,nm$.
• Mechanical Properties: Unique combination of stiffness, strength, and tenacity compared to other fiber materials.
• Conductivity: Extremely high thermal and electrical conductivity.
• Categorization:     - Single-wall nanotubes (SWNT): Single graphene layer. Structures include Armchair, Zig-zag, and Chiral, depending on the rolling angle.     - Multi-wall nanotubes (MWNT): Consists of several nested single-walled nanotubes with different diameters.
• Diverse Applications:     - Biological: Drug delivery, trapping dangerous substances, enzyme immobilization, and DNA transfection.     - Paints: Improving material strength and conductivity.     - Actuators: Converting electrical energy into mechanical energy (used in robotics).     - Electronics: Semiconductors and diodes.     - Chemical Industry: Catalysts, such as zeolites in hydrocarbon cracking.

Other Nanomaterial Types

Metal Based Materials

• Components: Includes quantum dots, nanogold, nanosilver, and metal oxides like $TiO_2$.
• Quantum Dots: Semiconductor crystals (size: few nm to few hundred nm) comprised of hundreds or thousands of atoms.

Dendrimers

• Etymology: From the Greek 'dendron' meaning "tree."
• Structure: Repetitively branched, nanosized polymers built from branched units. The surface contains numerous chain ends for specific chemical functions.
• Applications: Molecular recognition, nanosensing, light harvesting, opto-electrochemical devices, and drug delivery.

Composites

• Definition: Combinations of nanoparticles with other nanoparticles or with bulk-type materials.
• Mechanical Enhancement: Nanosized clays are added to auto parts or packaging to improve thermal and flame-retardant properties.
• Synergy: Formed of two or more materials with distinctive properties which act synergistically to create properties unattainable by a single material alone.