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Environmental Nanotechnology: An Introduction
1.1 Introduction
Nanoscience and nanotechnologies have enabled an understanding of matter and have profound implications in all sectors, i.e. agriculture and food, energy production efficiency, the automotive industry, cosmetics, medical and drugs, household appliances, computers and weapons. Nanoscience is multi-disciplinary and interdisciplinary branch of science and technology, which has an impact on virtually every spectrum of human endeavour including communications, computing, textiles, cosmetics, sports, therapy, automotive, environmental monitoring, fuel cells and energy devices, water purification, food and beverage industry, etc. The tiny objects constructed atom-by-atom or molecule-by-molecule present one of the exciting prospects for research in nanoscience.
The Royal Society (2004) and Royal Academy of Engineering gives the following definitions of ānano scienceā and ānanotechnologiesā:
Nanomaterials are structured components with at least one dimension less than 100 nm. Materials have one dimension in the nanoscale and are extended in the other two-dimensional layers, such as graphene, thin films or surface coatings. Materials that are at nano scale having two dimensions and extended to one dimension include nanowires and nanotubes. Materials that are nano scale in three dimensions are particles. Nanocrystalline materials such as precipitates, colloids and quantum dots (tiny particles of semiconductor materials), made up of nanometre-sized grains, fall in this category.
Nanoparticles can be defined as material purposefully produced with one dimension in 1ā100 nm range (as stated by the American Society for Testing and Materials (ASTM) Committee on Nanotechnology). The materials have unique properties compared to their bulk and atomic counterparts. The engineered nanoparticles are widely used by consumers as novel products. With the unique properties and characteristics such as their size and shape, it is possible that these materials have profound demand in the market. The use of nanoparticles in environmental technologies and the potential impact on the energy sector, potential effects on human health and the environment (adverse and beneficial) is reviewed by Biswas and Wu (2005). Environmental nanoparticles is a new and fast-growing field.
The principal factors that cause the properties of nanomaterials differ significantly from other materials: increased relative surface area and quantum effects. These factors can change or enhance properties such as reactivity, strength and electrical characteristics. As a particle decreases in size, a greater proportion of atoms are found at the surface compared to those inside. For example, a particle of size 30 nm has 5% of its atoms on its surface, that of size 10 nm, 20% of its atoms, and that of size 3 nm, 50% of its atoms. The nanoparticles have a much greater surface area per unit mass compared with larger particles, because growth and catalytic chemical reaction occur at surfaces, which means a given mass of material in the nanoparticulate form will be much more reactive than the same mass of material made up of larger particles. Nanomaterials can be nanoscale in one dimension (surface films), two dimensions (strands or fibres) or three dimensions (precipitates, colloids). They can exist in single, fused, aggregated or agglomerated forms with spherical, tubular and irregular shapes.
There is variety in the types of nanoparticles that have been fabricated, with almost every element in the periodic table, together with various alloys and compounds can form nanoparticles. Nanoparticles can be metallic, semiconducting or insulating and typically their properties are very different from those of the corresponding bulk materials. The seven main nanomaterial categories include carbon-based nanomaterials, nano-composites, nanometals and nano alloys; biological nanomaterials; nano-polymers; nano-glasses and nano-ceramics.
Due to their small dimension, nanomaterials have extremely large surface area to volume ratio, which gives rise to more āsurfaceā-dependent material properties. When the sizes of nanomaterial are comparable to length, the developed material will be affected due to surface properties of the nanomaterial. This will enhance or modify the properties of the bulk materials (e.g. metallic nanoparticles can be used as very active catalysts, chemical sensors and nanowires that enhance sensitivity and sensor selectivity). Different properties of nanomaterials have different applications in different areas (Table 1.1).
| Table 1.1 Nanoparticles Properties and Their Applications |
| Property | Application |
| Optical | Anti-reflection coatings. Tailored reflective index of surfaces. Light-based sensors for cancer diagnosis. |
| Magnetic | Increased density storage media. Nanomagnetic particles to create improved detail and contrast in MRI images. |
| Thermal | Enhance heat transfer from solar collectors to storage tanks. Improve efficiency of coolants in transformers. |
| Mechanical | Improved wear resistance. New anti-corrosion properties. New structural materials, composites, stronger and lighter. |
| Electronic | High performance and smaller components, e.g. capacitors for small consumer devices such as mobile phones. Displays that is cheaper, larger, brighter and more efficient. High conductivity materials. |
| Energy | High energy density and more durable batteries. Hydrogen storage applications using metal nanoclusters. Electrocatalysts for high efficiency fuel cells. Renewable energy, ultra high performance solar cells. Catalysts for combustion engines to improve efficiency, hence economy. |
| Biomedical | Anti-bacterial silver coatings on wound dressings. Sensors for disease detection (quantum dots). Programmed release drug delivery systems. āInteractiveā food and beverages that change colour, flavour or nutrients depending on a dinerās taste or health. |
| Environmental | Clean-up of soil contamination and pollution, e.g. oil. Biodegradable polymers. Aids for germination. Treatment of industrial emissions. More efficient and effective water filtration. |
| Surfaces | Dissolution rates of materials are highly size dependant. Activity of catalysts. Coatings for self-cleaning surfaces, Pilkingtonās glass for example. |
| Personal care | Effective clear inorganic sunscreens. |
1.2 Properties of Nanomaterials
1.Electrical properties: The electrical properties of nanomaterials vary between metallic and semiconducting materials and depend on the diameter of the nanomaterial. The very high electrical conductivity of nanomaterial is due to minimum defects in the structure.
2.Thermal conductivity: The thermal conductivity of nanomaterials is very high, due to the vibration of covalent bonds which is 10 times greater than the metal. The high thermal conductivity of nanomaterials is due to minimum defects in the structure.
3.Mechanical properties: Nanomaterials are very strong and withstand extreme strain. The synthetic method is used for producing nanomaterials that exhibit properties as result of their characteristic length scale being in the nanometre range (ā¼1ā100 nm). The synthetic method controls size in this range so that one property or another can be attained. Nanomaterials can be synthesized by two main methods ābottom upā and ātop downā.
The bottom-up approach involves the constitution of nanomaterials atom by atom, molecule by molecule and cluster by cluster. The chemical or biological methods are involved in the synthesis of nanostructured building blocks (e.g. nanoparticles) and subsequently assembled into final forms of nanomaterial in the bottom-up approach. The advantage of bottom-up approach is the possibility of obtaining nanostructures with lesser defects and more homogeneous chemical composition.
In top-down approach physical, chemical or mechanical methods are in...
