Nanomaterials: An Introduction

Nanomaterials are defined as functional substances with at least one dimension between 1 and 100 nm and a specific surface area by volume greater than 60 cm²/cm³.¹ They are used in a wide range of fields, including medicine, electronics, environmental remediation, and renewable energy.²

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What Is a Nanomaterial?

Nanotechnology is no longer a novel concept; it has evolved into a general-purpose technology. It has been used to generate various materials at the nanoscale, including passive and active nano-assemblies, general nanosystems, and small-scale molecular nanosystems.³

Nanomaterials can be produced from various materials, including polymers, ceramics, metals, and metal oxides, taking forms such as nanoparticles, nanotubes, and nanowires. These materials exhibit quantum confinement effects that restrict the movement of particles and electrons, resulting in unique properties.⁴

Types of Nanomaterials

Experts categorize nanomaterials into three dimensions: 0D, 1D, and 2D materials, each with distinct properties and specific applications.

0D Materials

Among the notable 0D nanomaterials used in various industries, quantum dots, nanoclusters, fullerenes, and nanoparticles are particularly popular. This class of nanomaterials has all three dimensions within the nanoscale threshold (10 nm).⁵

Quantum Dots

Quantum dots (QDs) are less than 10 nm in size and typically consist of a core made from semiconductor material and a shell made from a different material that acts as a surface passivation layer.

The specific structure of QDs is crucial for achieving the desired electrical and optical characteristics for various applications. Their small size grants them key attributes such as a high surface-to-volume ratio and quantized energy levels, as opposed to the continuous range of energy levels observed in bulk materials.

The optical properties of QDs are a key research focus, as they depend on factors such as size, composition, morphology, and surface chemistry. These properties can be adjusted in various ways, making QDs highly versatile for a wide range of applications.⁶

Fullerenes

Fullerenes are another prominent 0D nanomaterial, characterized by a conjugated system and a cage-like morphology. Defined as an allotrope of carbon, fullerenes feature interconnected rings of carbon atoms forming a mesh-like structure. They come in various geometrical shapes, including hollow spheres, tubular forms, and ellipsoids.⁷

Fullerenes are excellent conductors of electricity and heat, exhibiting remarkable tensile strength. While chemically stable, they are not entirely inert and can act as electrophiles in chemical reactions.

C₆₀, known as Buckminsterfullerene, is a highly stable type of fullerene with a structure resembling a soccer ball, composed of 60 carbon atoms arranged in twelve pentagons and twenty hexagons. This carbon allotrope showcases impressive chemical and physical properties, including non-toxicity, solubility, superconductivity, and unique photophysical characteristics.⁸

1D Materials

In 1D nanomaterials, one of the three dimensions exceeds 10 nm. Examples include nanofibers, nanowires, nanotubes, and nanorods, all of which are being utilized in various industrial applications.

Nanotubes

Among the most notable choices for industrial applications are TiO₂ and carbon nanotubes (CNTs), with CNTs currently attracting significant attention.

CNTs can be categorized into two regions: the sidewall and the tip. The tip resembles the structure of a fullerene hemisphere, making it more reactive.⁹ Depending on the number of concentric tubes in their structure, CNTs are classified as single (SWCNT), double, or multi-walled (MWCNTs).

A single-walled carbon nanotube (SWCNT) is essentially a rolled-up cylindrical graphene sheet, typically with diameters of only a few nanometers. In contrast, multi-walled carbon nanotubes (MWCNTs) have external diameters ranging from 2 to 100 nm, depending on the number of outer concentric tubes. The length of these nanotubes can vary from a few nanometers to several centimeters, allowing for aspect ratios as high as 1,000,000.¹⁰

Nanowires

Nanowires are very thin structures with diameters within the range of a few nanometers. They are often classified into two categories: crystalline and amorphous.

The fundamental properties of nanowires are primarily determined by their constituent material and length-to-diameter ratio. Carbon nanowires are created by heating graphite powder with nickel at approximately 1200 °C inside a quartz tube.

Silicon–carbon nanowires, produced through simulation methods, have shown single-crystalline hexagonal structures and high tensile strength. However, synthesizing nanowires in a highly reproducible manner remains a significant challenge.¹¹

2D Materials

Graphene and Mxenes are extensively utilized in various industries due to their advantageous properties, making them among the most popular 2D nanomaterials.

Graphene

Graphene is composed of a single layer of carbon atoms, with an estimated thickness of around 0.334 nm, the lowest among known materials. It boasts a superior specific surface area compared to its counterparts, notable electron mobility, exceptional thermal conductivity, and superior mechanical and tensile strength relative to other single-layer materials.¹²

Recently, many new materials similar to graphene have been developed, including black phosphorus (BP), layered double hydroxides (LDHs), transition metal halides (TMHs) such as PbI₂ and MgBr₂.¹³

MXenes

MXenes are two-dimensional materials derived from layered transition metal carbides, nitrides, or carbo-nitrides, created by selectively etching their parent MAX phases.

The chemical formula for the precursor to MXenes is M_n+1AX_n, where ‘M’ is an early transition metal, ‘A’ is an element from groups 13–15 of the periodic table, and ‘X’ represents either nitrogen or carbon. The value of ‘n’ ranges from 1 to 4.¹⁴

These materials possess excellent mechanical characteristics, flexible features, elasticity, and machinability, making them a core choice for various industrial applications.

Applications of Nanomaterials

Nanomaterials find extensive applications across various industries, particularly in the medical field, energy applications, and modern structures. 0D, 1D, and 2D nanomaterials all serve different purposes.

Quantum dots, characterized by higher electron mobility, are ideal for high-performance computing platforms and computer hardware. They are also utilized in photonic applications due to their high efficiency.⁶

CNTs have gained popularity in the biomedical industry, particularly as nanocarriers in drug delivery systems. The conjugation of CNTs with doxorubicin, a well-known anticancer drug, is a widely studied approach. Although the large diameter of CNTs leads to low cell uptake efficiency, it enhances drug adsorption. The release of doxorubicin from the surface of SWCNTs occurs in the acidic environments typical of tumor sites.¹⁵

Graphene is promising for use in field-effect transistors (FETs) and is commonly used as a reinforcement in coatings to enhance wear resistance. Its biocompatibility adds to the value of graphene derivatives in medicine and biology, with key applications in early virus diagnosis.

A unique feature of graphene oxide is its ability to convert stem cells into osteoblasts, or bone-generating cells. Additionally, porous 3D graphene-based scaffolds can be employed in bone repair through bone tissue engineering, highlighting graphene’s significance as an important nanomaterial in industrial applications.¹²

Overall, these materials possess inherently superior mechanical and electrical properties, making them a popular choice for diverse applications, particularly in biomedical and drug delivery domains. With ongoing advancements in materials science and technology, we can anticipate significant developments in the field of nanomaterials.

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References and Further Reading

1. Mekuye, B. et al. (2023). Nanomaterials: An overview of synthesis, classification, characterization, and applications. Nano Select. Available at: https://doi.org/10.1002/nano.202300038
2. Huynh, K., et al. (2020). Synthesis, Properties, and Biological Applications of Metallic Alloy Nanoparticles. International Journal of Molecular Sciences. Available at: https://doi.org/10.3390/ijms21145174
3. Malik, S., et al. (2023). Nanotechnology: A Revolution in Modern Industry. Molecules. 2023; 28(2):661. https://doi.org/10.3390/molecules28020661
4. Gupta, D., et al. (2023). Green and sustainable synthesis of nanomaterials: recent advancements and limitations. Environmental Research. Available at: https://doi.org/10.1016/j.envres.2023.116316
5. Joudeh, N., et al. (2022). Nanoparticle classification, physicochemical properties, characterization, and applications: a comprehensive review for biologists. J Nanobiotechnol. https://doi.org/10.1186/s12951-022-01477-8
6. Agarwal, K., et al. (2023). Quantum dots: An overview of synthesis, properties, and applications. Materials Research Express. https://doi.org/10.1088/2053-1591/acda17
7. Ekpete, O., et al. (2023). Fullerenes: synthesis and application. Faculty of Natural and Applied Sciences Journal of Scientific Innovations. https://www.fnasjournals.com/index.php/FNAS-JSI/article/view/137
8. Sam, M., et al. (2024). An overview of functionalized fullerenes and their applications in industry. Nano Science Technology. https://jnanoscitec.com/wp-content/uploads/2024/07/Volum-13-paper-1-1.pdf
9. Ijaz, H., et al. (2023). Review on carbon nanotubes (CNTs) and their chemical and physical characteristics, with particular emphasis on potential applications in biomedicine. Inorganic Chemistry Communications. https://doi.org/10.1016/j.inoche.2023.111020
10. Sacco L. et al. (2023). Overview of Engineering Carbon Nanomaterials Such As Carbon Nanotubes (CNTs), Carbon Nanofibers (CNFs), Graphene and Nanodiamonds and Other Carbon Allotropes inside Porous Anodic Alumina (PAA) Templates. Nanomaterials. https://doi.org/10.3390/nano13020260
11. Hsu, C., et al. (2023). Nanowires properties and applications: a review study. South African Journal of Chemical Engineering. https://doi.org/10.1016/j.sajce.2023.08.006
12. Urade, A. et al. (2023). Graphene Properties, Synthesis and Applications: A Review. JOM 75. https://doi.org/10.1007/s11837-022-05505-8
13. Uddin, M., et al. (2023). Graphene-like emerging 2D materials: recent progress, challenges and future outlook. RSC advances. https://doi.org/10.1039/D3RA04456D
14. Akhter, R., et al. (2023). MXenes: Acomprehensive review of synthesis, properties, and progress in supercapacitor applications. Journal of Materiomics. https://doi.org/10.1016/j.jmat.2023.08.011
15. Yahyazadeh, A., et al. (2024). Carbon Nanotubes: A Review of Synthesis Methods and Applications. Reactions. https://doi.org/10.3390/reactions5030022

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