Chapter 16: Green Chemistry and Nanochemistry
In December 1959, at a meeting of physicists at Caltech, Richard Feynman gave a talk with an unusual title: “There’s Plenty of Room at the Bottom.” His question was simple and startling. Nothing in the laws of nature, he argued, forbids us from arranging atoms one at a time, exactly where we want them. If we could learn to do that, an enormous amount of information could be stored in an almost invisible space, and materials could be built to order rather than merely dug out of the ground. Most of his listeners treated the idea as an entertaining thought experiment. It was not. Within a few decades, the scanning tunnelling microscope made it possible not only to see individual atoms on a surface but to push them into chosen positions — one such arrangement, a ring of iron atoms on copper, appears later in this chapter.
While that world of the very small was opening up, chemists were facing a very different problem at the other end of the scale: entire industries producing useful substances along with mountains of hazardous waste. Their answer was not a bigger clean-up plant but a change in design — build the waste out of the reaction in the first place. This chapter follows both ideas: chemistry made cleaner, and chemistry made smaller.
Diagram
Fig. 16.0 – Split illustration: left half shows a factory redesigned for zero waste with a recycling arrow loop and a green leaf motif; right half shows a magnified view descending from a hand, to a human hair, to a red blood cell, to a cluster of atoms labelled 1–100 nm
Keep these questions in mind as you read — the chapter will give you everything you need to reason out each answer.
16.1 Introduction
Chemistry has done a great deal to improve the quality of our lives. The medicines that cure us, the fertilizers that raise crop yields, the fuels that move vehicles, the plastics, dyes, detergents and fibres we use every day — all of these come from chemical knowledge put to work on a large scale.
Unfortunately, the same success has brought a cost with it. Our health and the global environment are now under threat. Along with a rapidly growing human population and the spread of industry since the industrial revolution, two problems stand out as major global concerns of the 21st century: the energy crisis and environmental pollution.
For a long time the usual response to pollution was to deal with it after it appeared — treat the effluent, scrub the chimney gas, bury or burn the waste. This is expensive, it is never complete, and it does nothing about the hazardous substances that were used inside the factory in the first place. To reduce the problems of the energy crisis and pollution, chemists therefore had to change the question they were asking: instead of “how do we clean this up?”, they began to ask “how do we design the process so that the harmful substance is never made at all?” This change of approach is called green chemistry, and the difference between the two strategies is summarised in FC 16.1.
Flowchart
FC 16.1 – Two responses to chemical pollution: conventional route: raw materials → process → product + hazardous waste → treatment plant / disposal → residual pollution; green route: redesigned choice of raw materials and conditions → process → product + little or no hazardous waste → nothing left to treat
What Green Chemistry Means
Green chemistry is an approach to chemistry that aims to maximize efficiency and to minimize hazardous effects on human health and the environment. Here “efficiency” does not only mean a high yield. It also means that as much as possible of the material we start with should end up in the product we want, that as little energy as possible should be spent, and that whatever does leave the process should be harmless.
The concept of green chemistry was coined by Paul T. Anastas.
Read that definition carefully, because two words in it carry the whole idea. The word design tells us that green chemistry acts at the planning stage of a reaction or a product, not at the disposal stage. The word prevention tells us the goal is to stop the hazard from arising, not to manage it afterwards.
Why a Set of Principles Is Needed
Wanting a cleaner process is not the same as knowing how to build one. A chemist redesigning a synthesis has to make dozens of decisions — which starting material, which solvent, which catalyst, what temperature, how many steps — and each of these can make the process cleaner or dirtier.
For this reason, to reduce the impact of the energy crisis and pollution and to save natural resources, we need to implement the twelve principles of green chemistry enunciated by Paul Anastas wherever possible. These twelve principles, taken up one by one in Section 16.3, act as a checklist against which any chemical process can be judged and improved.
16.2 Sustainable Development
Green chemistry plays an important role in sustainable development. In fact, the two ideas support each other: sustainable development sets the goal, and the twelve principles of green chemistry give chemists a practical way of reaching it. We can achieve sustainable development by adapting the twelve principles of green chemistry.
Read the definition as a balance between two claims. The people living today have real needs — food, medicines, fuel, housing, employment — and development must meet them. But the coal, petroleum, ores and clean water used to meet those needs are drawn from a limited stock, and whatever is used up or spoiled today is simply not available to anyone later. Development is sustainable only when both halves of that sentence are satisfied at the same time.
Sustainable development has continued to evolve as the idea of protecting the world’s resources. The pattern to be avoided is the straight line of take, make and throw away; the pattern to be aimed at is a loop, in which materials are used again and the environment is handed on undamaged. The two patterns are compared in FC 16.2.
Flowchart
FC 16.2 – Two patterns of resource use: linear pattern: extract raw material → manufacture → use → discard as waste → resource lost and environment burdened; circular (sustainable) pattern: extract raw material → manufacture with minimum waste → use → recover and recycle → back to manufacture, with only a small input of fresh raw material
16.3 Principles of Green Chemistry
The twelve principles listed below are the working tools of green chemistry. Each one points to a decision a chemist actually makes while planning a reaction — what to start with, what to dissolve it in, how hot to run it, how many steps to use, and what will happen to the product once it is thrown away. Taken together they form a checklist: the more of them a process satisfies, the greener it is.
Principle 1: Prevention of Waste
The first principle is the simplest and the most important. Priority should be given to the prevention of waste rather than to cleaning up and treating waste after it has been created.
Illustration: The aim is to develop zero waste technology (ZWT). In terms of ZWT, in a chemical synthesis the waste product should be zero or minimum. Zero waste technology also aims to use the waste product of one system as the raw material for another system, so that nothing leaves the boundary of the industry as useless material.
For example: (1) bottom ash from a thermal power station can be used as a raw material for the cement and brick industry; (2) effluent coming out from the cleansing of machinery parts may be used as coolant water in a thermal power station.
Notice what has happened in both examples. The material has not been destroyed or buried — it has simply been redefined. What one plant calls waste, another plant calls feedstock, and the disposal problem disappears along with the cost of disposal.
Principle 2: Atom Economy
Yield tells us how much of the desired product we obtained compared with the maximum possible. It says nothing at all about how many atoms we had to bring into the flask to get it. Atom economy answers that second question.
Good atom economy means most of the atoms of the reactants are incorporated in the desired product, and only small amounts of unwanted by-products are formed — and hence there are lesser problems of waste disposal.
Illustration: The concept of atom economy gives the measure of the unwanted product produced in a particular reaction. It is calculated as
\[\%\ \text{atom economy} = \frac{\text{Formula weight of the desired product}}{\text{Sum of formula weights of all the reactants used in the reaction}} \times 100\]
Here “formula weight” means the mass of one formula unit in unified atomic mass units (u), obtained by adding the atomic masses of all the atoms in the formula. Every reactant that is consumed goes into the denominator, even the ones we normally think of as helpers.
For example: the conversion of butan-1-ol to 1-bromobutane.
\[\mathrm{CH_3CH_2CH_2CH_2OH} + \mathrm{NaBr} + \mathrm{H_2SO_4} \longrightarrow \mathrm{CH_3CH_2CH_2CH_2\!-\!Br} + \mathrm{NaHSO_4} + \mathrm{H_2O}\]
\[\%\ \text{atom economy} = \frac{\text{mass of }(4\mathrm{C} + 9\mathrm{H} + 1\mathrm{Br})\ \text{atoms}}{\text{mass of }(4\mathrm{C} + 12\mathrm{H} + 5\mathrm{O} + 1\mathrm{Br} + 1\mathrm{Na} + 1\mathrm{S})\ \text{atoms}} \times 100\]
\[= \frac{137\ \text{u}}{275\ \text{u}} \times 100 = 49.81\ \%\]
The numerator counts only the atoms that appear in the desired product, 1-bromobutane. The denominator counts every atom supplied by butan-1-ol, sodium bromide and sulphuric acid together. Fewer than half the atoms brought into this reaction end up in the product we wanted; the rest leave as sodium hydrogen sulphate and water.
Principle 3: Less Hazardous Chemical Synthesis
Designed chemical reactions and synthesis routes should be as safe as possible, so that we can avoid the formation of hazardous waste from chemical processes.
Illustration: Earlier, DDT (dichlorodiphenyltrichloroethane) was used as an insecticide, and it was effective in controlling diseases like typhoid and malaria carried by mosquitoes. It was later realized that DDT is harmful to living things. Nowadays benzene hexachloride (BHC) is used as an insecticide. One of the isomers of BHC, the γ-isomer (gamma), is called gammexane or lindane.
Principle 4: Designing Safer Chemicals
This principle is quite similar to the previous one. The aim is to develop products that are less toxic, or which require less toxic raw materials.
Illustration: In chemical industries, workers are exposed to a toxic environment. In order to prevent the workers from exposure to toxicity, we should think of designing safer chemicals.
For example, adipic acid is widely used in the polymer industry. Benzene is the starting material for the synthesis of adipic acid, but benzene is carcinogenic, and benzene, being a volatile organic compound (VOC), pollutes air. In green technology developed by Drath and Frost, adipic acid is enzymatically synthesised from glucose.
The change here is worth noticing: the product, adipic acid, is exactly the same molecule as before. What has been made safer is the raw material and the route — a sugar and an enzyme in place of a carcinogenic hydrocarbon.
Principle 5: Use of Safer Solvents and Auxiliaries
Choose the safer solvent available for any given step of a reaction. Minimize the total amount of solvents and auxiliary substances used, as these make up a large percentage of the total waste created.
Illustration: The main aim behind this principle is to use green solvents — for example, water and supercritical \(\mathrm{CO_2}\) in place of volatile halogenated organic solvents such as \(\mathrm{CH_2Cl_2}\), \(\mathrm{CHCl_3}\) and \(\mathrm{CCl_4}\), for chemical synthesis and other purposes.
Solvents are chemicals that dissolve solutes and form solutions, and they facilitate many reactions. Water is a safe, benign solvent, while dichloromethane is hazardous. The use of toxic solvents affects millions of workers every year and has implications for consumers and for the environment as well.
Many solvents are used in high volumes and many of them are volatile organic compounds. Their use creates large amounts of waste, air pollution and other health impacts. Finding safer, more efficient alternatives, or removing solvents altogether, is one of the best ways to improve a process or a product.
Principle 6: Design for Energy Efficiency
Chemical synthesis should be designed to minimize the use of energy. It is better to minimize the energy by carrying out reactions at room temperature and pressure.
This can be achieved by the use of a proper catalyst, the use of micro-organisms for organic synthesis, the use of renewable materials, and so on.
Illustration: A biocatalyst can work at ambient conditions. Similarly, in chemical synthesis, refluxing conditions require less energy, improving the technology of the heating system, using microwave heating, and so on.
The reason energy matters so much is that most industrial heat and electricity still comes from burning fuel. A reaction that runs at room temperature rather than at 500 K is therefore not only cheaper — it also releases less carbon dioxide into the atmosphere, even though no carbon dioxide appears anywhere in its equation.
Principle 7: Use of Renewable Feedstocks
The perspective of this principle is largely towards petrochemicals. Use chemicals which are made from renewable (plant-based) sources rather than from other sources — for example, crude oil.
Illustration: Overexploitation of non-renewable feedstocks will deplete the resources, and future generations will be deprived of them. Moreover, the use of these non-renewable resources puts a burden on the environment.
On the other hand, the use of renewable resources — for example agricultural or biological products — ensures the sharing of resources by future generations. This practice generally does not put much burden on the environment. The products and waste from such feedstocks are generally biodegradable.
This principle is where green chemistry and sustainable development meet most directly, because a renewable feedstock is grown again within a human lifetime, while crude oil is not.
Principle 8: Reduce Derivatives (Minimization of Steps)
A commonly used technique in organic synthesis is the use of a protecting or blocking group. A protecting group is attached to a sensitive functional group so that it survives untouched while some other part of the molecule is altered; afterwards it must be removed again.
Unnecessary derivatization — for example, the installation and removal of protecting groups — should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
Illustration: In organic synthesis, we very often need protection of some functional groups, and finally we again need their deprotection. This is explained in the following example of the synthesis of m-hydroxybenzoic acid from m-hydroxybenzaldehyde, drawn in Fig. 16.16.
Diagram
Fig. 16.16 – Three-step synthesis of m-hydroxybenzoic acid from m-hydroxybenzaldehyde: benzene ring with CHO and OH groups in meta positions → arrow labelled C₆H₅CH₂Cl, “protection of –OH group” → ring with CHO and OCH₂C₆H₅ → arrow labelled [O] → ring with COOH and OCH₂C₆H₅ → downward arrow labelled “deprotection of OH group” → ring with COOH and OH, labelled (m-hydroxybenzoic acid)
The oxidation of the –CHO group to –COOH is the only step that was actually wanted. The first and third steps exist only to keep the phenolic –OH group out of the way while the oxidation is carried out, and the sequence is summarised in FC 16.3.
Flowchart
FC 16.3 – Protection–reaction–deprotection sequence: m-hydroxybenzaldehyde → protection of –OH using C₆H₅CH₂Cl → protected aldehyde (OCH₂C₆H₅) → oxidation with [O] → protected acid → deprotection of the OH group → m-hydroxybenzoic acid; side note: only the oxidation step changes the target functional group, the other two steps are extra
Obviously, in such cases the atom economy is also less, because each extra step brings in reagents whose atoms never appear in the final product. The green chemistry principle aims to develop methodology where unnecessary steps should be avoided; if practicable, biocatalytic reactions very often need no protection of a selective group, since an enzyme attacks only the group it is designed to act on.
Principle 9: Use of Catalysis
The use of a catalyst in a chemical reaction speeds up its rate. A catalyst helps to increase selectivity, minimize waste, and reduce reaction times and energy demands.
The word “selectivity” carries a lot of weight here. A well-chosen catalyst pushes the reaction towards one product rather than a mixture, which means less separation work afterwards and less material discarded as the wrong product. And because a catalyst is regenerated, a small quantity serves through the whole run instead of being consumed batch after batch.
| Reaction | Name of catalyst used |
|---|---|
| 1. Hydrogenation of oil (hardening) | ………………… |
| 2. Haber’s process for the manufacture of ammonia | ………………… |
| 3. Manufacture of HDPE polymer | ………………… |
| 4. Manufacture of \(\mathrm{H_2SO_4}\) by the contact process | ………………… |
| 5. Fischer–Tropsch process (synthesis of gasoline) | ………………… |
Principle 10: Design for Degradation
Design chemicals that degrade and can be discarded easily. Ensure that both the chemicals and their degradation products are not toxic, bioaccumulative or environmentally persistent.
Three words in that statement need unpacking. A persistent substance stays chemically unchanged in soil or water for years. A bioaccumulative substance builds up inside the bodies of living organisms faster than it is removed, so its concentration rises as it passes up a food chain. A substance that is both persistent and bioaccumulative causes damage far from where it was used, and long after it was used.
Illustration: The aim behind this principle is that the waste product should degrade automatically to clean the environment. Thus, biodegradable polymers and pesticides are always preferred. To make separation easier for the consumer, an international plastic recycle mark is printed on larger items, as shown in Fig. 16.17.
Diagram
Fig. 16.17 – The seven international plastic recycling codes: the triangular chasing-arrows symbol with the numbers 1 to 7 inside, labelled PETE, HDPE, V (PVC), LDPE, PP, PS and Other, arranged around a central label “Household plastics”
Use this chart to sort plastic materials in daily life.
| Code | Polymer | Used in | Status | Is recycled to make |
|---|---|---|---|---|
| 1 | PETE or PET (polyethylene terephthalate) | Microwavable food trays; salad dressing, soft drink, water and beer bottles | Hard to clean; absorbs bacteria and flavours; avoid reusing | Carpet, furniture, new containers, polar fleece |
| 2 | HDPE (high density polyethylene) | Household cleaner and shampoo bottles | Transmits no known chemicals into food | Detergent bottles, fencing, floor tiles, pens |
| 3 | V or PVC (vinyl) | Cooking oil bottles, clear food packaging, mouthwash bottles | Believed to contain phthalates that interfere with hormonal development; avoid | Cables, mudflaps, panelling, roadway gutters |
| 4 | LDPE (low density polyethylene) | Bread and shopping bags, carpet, clothing, furniture | Transmits no known chemicals into food | Envelopes, floor tiles, lumber, trash-can liners |
| 5 | PP (polypropylene) | Ketchup bottles, medicine and syrup bottles, drinking straws | Transmits no known chemicals into food | Battery cables, brooms, ice scrapers, rakes |
| 6 | PS (polystyrene) | Disposable cups and plates, egg cartons, take-out containers | Believed to leach styrene, a possible human carcinogen, into food; avoid | Foam packaging, insulation, light switchplates, rulers |
| 7 | Other (miscellaneous) | 3- and 5-gallon water jugs, nylon, some food containers | Contains bisphenol A, which has been linked to heart disease and obesity; avoid | Custom-made products |
Principle 11: Real-time Analysis for Pollution Prevention
Analytical methods need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
Illustration: Analytical methodologies should be developed or modified so that continuous monitoring of manufacturing and processing units is possible. This is very much important for the chemical industries and for nuclear reactors.
The phrase “real-time” is the key. Testing a sample in the laboratory after the batch is finished tells us what went wrong; a sensor reading the reaction mixture continuously lets the operator correct the temperature or the feed rate while the batch is still running, before a hazardous substance has had a chance to build up.
Principle 12: Safer Chemistry for Accident Prevention
We need to develop chemical processes that are safer and that minimize the risk of accidents.
Illustration: The substances to be used in a chemical reaction should be selected in such a way that they can minimize the occurrence of chemical accidents, explosions, fire and emission. For example, if the chemical process works with gaseous substances, then the possibility of accidents including explosion is relatively higher compared to a system working with non-volatile liquid and solid substances.
The reason is physical rather than chemical. A gas expands to fill whatever space it escapes into and mixes rapidly with air, so a leak spreads within seconds; a non-volatile liquid or a solid stays broadly where it was spilt and can be contained.
16.4 The Role of Green Chemistry
The green chemistry approach begins with an honest observation about the planet. The Earth does have a natural capacity for dealing with much of the waste and pollution that society generates — rivers dilute, soil bacteria break down organic matter, and the atmosphere disperses and oxidises many gases. It is only when that capacity is exceeded that we become unsustainable.
This is why the question is never simply “is this substance harmful?” but “how much of it are we releasing, and can the environment still handle that much?” Once the rate of release runs ahead of the rate at which nature can absorb and break down the material, the excess accumulates — and accumulation is what turns a discharge into pollution. The two possibilities are set out in FC 16.4.
Flowchart
FC 16.4 – Assimilative capacity of the environment: waste released into air, water or soil → compare with the natural rate of dispersal and breakdown → branch: if rate of release ≤ natural capacity → waste is degraded, no build-up, system stays sustainable; if rate of release > natural capacity → excess accumulates → pollution, damage to health and ecosystems, unsustainable
What Green Chemistry Sets Out to Do
The purpose of green chemistry is to promote innovative chemical technologies that reduce or eliminate the use or generation of hazardous substances in the design, manufacture and use of chemical products. Notice that all three stages are named — the hazard is examined when the product is being planned, when it is being made, and when it is finally used by a consumer.
Green chemistry helps to reduce capital expenditure and to prevent pollution. A plant that generates little hazardous waste needs less treatment machinery, less storage for hazardous material and less expenditure on disposal, so cleaner design is usually cheaper design in the long run.
Green chemistry incorporates pollution prevention practices in the manufacture of chemicals, and promotes pollution prevention and industrial ecology. Industrial ecology means treating a group of industries the way we treat a natural ecosystem, in which the output of one organism becomes the input of another, so that materials keep circulating instead of piling up.
In short, green chemistry is a new way of looking at chemicals and at their manufacturing processes, in order to minimize any negative environmental effects. Right now the green chemistry revolution is only beginning, and it is an exciting time, with new challenges for chemists involved with the discovery, manufacture and use of chemicals.
Two Global Problems It Addresses
Green chemistry helps to protect the presence of ozone in the stratosphere, which is essential for the survival of life on the Earth. The stratospheric ozone layer absorbs a large part of the ultraviolet radiation coming from the Sun, and this radiation would otherwise damage living tissue. Substances that reach the stratosphere and destroy ozone therefore have to be designed out of use altogether, which is exactly what green chemistry asks of a product designer.
Green chemistry is also useful in controlling the greenhouse effect, that is, global warming. Gases such as carbon dioxide allow sunlight to pass through the atmosphere but absorb the heat radiated back from the Earth’s surface, so a rising concentration of these gases raises the average surface temperature. Processes that use less energy release less of these gases, which is why the energy-efficiency principle matters far beyond the walls of the factory.
So we should think about saving the environment and saving the Earth.
The recovery of the ozone layer after the phase-out of these gases is one of the clearest tests of whether such action works, and Fig. 16.18 shows the record.
Diagram
Fig. 16.18 – Line graph with year on the x-axis (1980 to the present) and two curves: global emission of ozone-depleting substances (falling sharply after 1987) and measured stratospheric ozone level (falling to a minimum, then slowly recovering); mark the year of the Montreal Protocol with a vertical dashed line
16.5 Introduction to Nanochemistry
From the clothes and sunglasses you wear to computer hard drives and even cleaning products, nanotechnology plays a big role in the manufacture of many materials. We have been using lasers in DVD and CD players for a long time, and these contain nanosize components.
Look at Fig. 16.1, which shows comparative scales running from macro-materials all the way down to atoms.
Diagram
Fig. 16.1 – Horizontal logarithmic scale in nm from 0.1 to 100,000,000, with objects placed along it: water molecule, glucose, antibody, virus, bacteria, cancer cell, a printed period, tennis ball
Also observe Fig. 16.2, which depicts materials found in nature as well as devices that are man-made.
Diagram
Fig. 16.2 – Scale of nanomaterials from 10⁻² m down to 10⁻⁷ m, with labelled examples at each level: ant 5 mm, dust mite 200 µm, human hair 10–50 µm wide, microelectromechanical devices 10–100 µm, red blood cells 2–5 µm wide, quantum corral of 48 iron atoms on a copper surface (14 nm corral diameter), DNA 2–12 nm diameter, carbon nanotube 2 nm diameter
In both figures, some objects — the tennis ball in Fig. 16.1, and the ant and the human hair in Fig. 16.2 — we can see with our own eyes, whereas bacteria, viruses and red blood cells we cannot observe with the naked eye. Materials whose structures lie in this extremely small range are known as nanomaterials.
What Is Nanoscience?
At very small sizes, matter stops behaving the way we expect it to. A metal may change colour, a hard solid may become harder still, an unreactive element may turn into a good catalyst. Studying these changes is the business of nanoscience.
Defining Nanotechnology
Nanoscience asks what happens at this scale; nanotechnology puts the answer to use. It covers the whole route from an idea to a usable device — deciding the structure, checking what has actually been made, producing it, and applying it.
Why Nano?
The nanometer scale needs to be fixed clearly before anything else. Conventionally, the nanometer scale is defined as 1–100 nm. One nanometer is one billionth of a metre, that is,
\[1\ \text{nm} = 10^{-9}\ \text{m}\]
To get a feel for how small this is, remember that a metre contains a thousand million nanometres. A human hair, which we can just about see across, is tens of thousands of nanometres wide.
The materials we see around us are bulk materials that possess macroscopic physical properties. A grain of sand, which is a micron-sized material, still possesses the same bulk properties as a large block of the same substance — grinding sand finer does not change what sand is.
But a material synthesized at the nanoscale (1 nm – 100 nm) possesses unique optical, structural, thermal, catalytic, magnetic and electrical properties. These properties change as a function of size and are very different from those of their bulk materials. This is the whole reason the field exists: at the nanoscale, size itself becomes a property that can be adjusted, just as composition can.
What Is a Nanomaterial?
Read the phrase “at least one dimension” carefully, because it is what makes the definition broad. A sheet only a few nanometres thick counts as a nanomaterial even though it may be centimetres wide, because one of its dimensions lies in the nanoscale.
Nanomaterials are larger than single atoms but smaller than bacteria and cells. These may be nanoparticles, nanowires and nanotubes, according to their dimensions. Nanostructured materials may be large organic molecules, inorganic cluster compounds, and metallic or semiconductor particles.
Zero-, One- and Two-Dimensional Nanostructures
Nanostructures are classified by counting how many of their dimensions lie in the nanoscale.
Zero-dimensional nanostructures: for example, nanoparticles. A zero-dimensional structure is one in which all three dimensions are in the nanoscale.
One-dimensional nanostructures: for example, nanowires and nanorods. A one-dimensional nanostructure is one in which two dimensions are in the nanoscale.
Two-dimensional nanostructures: for example, thin films. A two-dimensional nanostructure is one in which one dimension is in the nanoscale.
The types of material belonging to each class are illustrated in Fig. 16.3.
Diagram
Fig. 16.3 – Illustration of zero, one, two dimensions: three small block drawings, one with a tiny dot on the face (particle), one with a thin line along an edge (wire), one with a shaded top face (thin film), each beside its dimension label
| Nanomaterial dimension | Nanomaterial type | Example |
|---|---|---|
| All three dimensions < 100 nm | Nanoparticles, quantum dots, nanoshells, nanorings, microcapsules | Zero-dimensional structure |
| One dimension < 100 nm | Nanotubes, fibres, nanowires | One-dimensional structure |
| Two dimension < 100 nm | Thin films, layers and coatings | Two-dimensional structure |
Defining Nanochemistry
Chemistry contributes the actual making of these materials — choosing the reactants, controlling the conditions, and directing how the tiny pieces assemble.
Nanochemistry is used in chemical, physical and materials science, as well as in engineering, biological and medical applications.
The word “architectures” is well chosen. In nanochemistry it is not enough to make particles of the right size; they then have to be arranged into a structure that performs a job — a coating, a sensor, a catalyst bed or a drug carrier.
16.6 Characteristic Features of Nanoparticles
What makes the science at the nanoscale special is that at such a small scale, different laws dominate over those that we experience in our everyday life.
There is a simple physical reason behind almost everything in this section. In a large piece of matter, the overwhelming majority of atoms sit in the interior, completely surrounded by neighbours, and the few atoms on the surface hardly matter. As the particle shrinks, the fraction of atoms lying on the surface grows rapidly, and surface atoms have unsatisfied bonds and higher energy than interior atoms. Once a sizeable share of all the atoms are surface atoms, the material’s colour, reactivity, melting point, strength and conductivity all begin to shift.
16.6.1 Colour
Colour is an optical property that is different at the nanoscale. Elemental gold, as we know, has a nice shining yellow colour. However, if you had only 100 gold atoms arranged in a cube, its colour would be red.
Colour arises from the way a material interacts with light. In a metal, light is absorbed and re-emitted by the loosely held electrons of the metallic bond. When the particle becomes smaller than the wavelength of visible light, those electrons are confined within the particle, and the wavelength they absorb most strongly depends on how large that confining space is. Change the particle size and you change the absorbed wavelength — which means you change the colour of the sample, without changing the element at all. Fig. 16.4 shows solutions of gold nanoparticles of different sizes.
Diagram
Fig. 16.4 – Formation of gold nanoparticles solution: three glass vials labelled A, B and C standing side by side, holding colloidal gold of increasing particle size and showing pale yellow, red and deep purple-red colours respectively
16.6.2 Surface Area
A high surface-to-volume ratio is a very important characteristic of nanoparticles. If a bulk material is subdivided into a group of individual nanoparticles, the total volume remains the same, but the collective surface area is largely increased.
With a large surface area for the same volume, these small particles react much faster, because more surface area provides a greater number of reaction sites, leading to more chemical reactivity. This is the same reason a powdered solid dissolves faster than a lump of it — carried to an extreme.
The explanation of the increase in surface area with a decrease in particle size is shown in Fig. 16.5.
Diagram
Fig. 16.5 – Surface area of nanoparticles: a solid cube of edge 1 m labelled “Area = 6 × 1m² = 6 m²”; the same cube divided into 8 cubes of edge ½ m labelled “Area = 6 × (1/2m)² × 8 = 12 m²”; and divided into 27 cubes of edge ⅓ m labelled “Area = 6 × (1/3m)² × 27 = 18 m²”
Fig. 16.5 shows the surface areas when a cube of \(1\ \text{m}^3\) was progressively cut into smaller cubes, until cubes of \(1\ \text{nm}^3\) were formed. The pattern is clear from the three stages drawn there: the material and its total volume never change, but every cut creates new faces, and the total exposed area keeps climbing.
16.6.3 Catalytic Activity
Due to the increase in surface area with a decrease in particle size, nanomaterial-based catalysts show increased catalytic activity. Usually they are heterogeneous catalysts, which means the catalysts are in the solid form and the reactions occur on the surface of the catalyst.
Since the reaction happens only on the surface, a catalyst that offers more surface per gram is a better catalyst per gram. Nanoparticle catalysts can also be easily separated from the reaction mixture and can be recycled, which keeps the process both cheaper and cleaner. For example, Pd and Pt metal nanoparticles are used in hydrogenation reactions.
\(\mathrm{TiO_2}\) and \(\mathrm{ZnO}\) are used in photocatalysis, that is, in reactions driven by absorbed light. Gold in bulk form is unreactive, but gold nanoparticles are found to be very good catalysts for various organic reactions.
16.6.4 Thermal Properties: Melting Point
The melting point of a nanomaterial changes drastically and depends on size. For example, sodium clusters (\(\mathrm{Na}_n\)) of 1000 atoms appeared to melt at 288 K, while a cluster of 10,000 atoms melted at 303 K, and bulk sodium melts at 371 K.
Melting means that the ordered arrangement of atoms breaks down. An atom in the interior of a crystal is held by neighbours on all sides and needs a great deal of thermal energy to break free; an atom on the surface is held by fewer neighbours and is far easier to dislodge. In a tiny cluster, a large share of the atoms are surface atoms, so the whole particle loses its order at a lower temperature.
Diagram
Fig. 16.19 – Graph of melting temperature against number of atoms in a sodium cluster: points at 1000 atoms (288 K) and 10,000 atoms (303 K) joined by a rising curve that flattens towards a horizontal dashed line marked “bulk sodium, 371 K”
16.6.5 Mechanical Properties
Mechanical strength: Nanosized copper and palladium clusters with diameters in the size range of 5–7 nm can have hardness up to 500% greater than that of the bulk metal.
A metal is normally deformed when planes of atoms slide over one another past defects in the crystal. When the grains of the metal are only a few nanometres across, there is very little room for such sliding to spread, so the material resists deformation far better and is measured as being much harder.
16.6.6 Electrical Conductivity
Electrical conductivity is observed to change at the nanoscale. For example, a carbon nanotube can act as a conductor or as a semiconductor in behaviour.
What decides which of the two it is, is the geometry of the tube — the direction in which the sheet of carbon atoms is rolled up, and the diameter of the resulting tube. Two nanotubes made of nothing but carbon atoms can therefore behave as differently as a metal wire and a silicon chip, which is why they are of such interest to the electronics industry.
16.7 Synthesis of Nanomaterials
16.7.1 Approaches to Synthesis
There are two approaches to the synthesis of nanomaterials: bottom up and top down. Fig. 16.6 shows a schematic illustration of the preparation methods of nanoparticles.
Diagram
Fig. 16.6 – Schematic illustration of the preparation of nanoparticles: a downward broad arrow from a block labelled “Bulk” through progressively smaller fragments to a central label “Nanoparticles” marked Top-Down; an upward broad arrow from scattered atoms and clusters to the same central label marked Bottom-up
In the bottom up approach, molecular components arrange themselves into more complex assemblies — atom by atom, molecule by molecule and cluster by cluster — building upward from the bottom. Example: synthesis of nanoparticles by colloidal dispersion.
In the top-down approach, nanomaterials are synthesized from bulk material by breaking the material. The bulk solids are dis-assembled into finer pieces until they are constituted of only a few atoms.
The two routes therefore start from opposite ends of the size scale and meet in the middle. Building up allows very fine control over size and shape, since the particle grows only as long as we let it grow; breaking down is simpler and better suited to large quantities, but the pieces obtained vary more in size.
16.7.2 Wet Chemical Synthesis of Nanomaterials
Sol-gel process: two terms must be fixed before the process itself makes sense.
Sols are dispersions of colloidal particles in a liquid. Colloids are solid particles with diameters of 1–100 nm — that is, the colloidal size range and the nanoscale are the same range, which is why colloid chemistry became the natural starting point for making nanomaterials.
A gel is an interconnected rigid network with pores of submicrometer dimensions, and polymeric chains whose average length is greater than a micrometer. In simple terms, the separate particles of the sol have joined into one continuous framework, with the liquid trapped inside its pores, so the mixture no longer flows.
A sol-gel process is based on inorganic polymerization reactions. It is generally carried out at room temperature and includes four steps: hydrolysis, polycondensation, drying and thermal decomposition. This method is widely employed to prepare oxide materials.
The whole route is drawn in Fig. 16.7, which also shows the different products obtained depending on how the gel is treated.
Diagram
Fig. 16.7 – Schematic representation of the sol-gel process: precursor solution → sol → (dehydration reaction) → gel → (rapid drying) → aerogel; from the sol, a spinning branch → thin film coating (calcine); from the gel, branches → powder (calcine) and xerogel → dense ceramic (calcine)
The Reactions Involved
The reactions involved in the sol-gel process can be described as follows.
\[\mathrm{MOR} + \mathrm{H_2O} \longrightarrow \mathrm{MOH} + \mathrm{ROH} \qquad \text{(hydrolysis)}\]
Here MOR is a metal alkoxide: M stands for the metal atom and OR for an alkoxide group. Water attacks the metal alkoxide and replaces an OR group by an OH group, releasing the corresponding alcohol, ROH.
\[\mathrm{MOH} + \mathrm{ROM} \longrightarrow \mathrm{M\!-\!O\!-\!M} + \mathrm{ROH} \qquad \text{(condensation)}\]
In the condensation step, an M–OH group and an M–OR group join to form an M–O–M linkage, again releasing alcohol. Repeating this linking step again and again is what builds the extended metal–oxygen network — which is why the process is described as an inorganic polymerization.
Stages of the Sol-Gel Process
- Formation of a stable solution of the alkoxide or of the solvated metal precursor.
- Gelation, resulting from the formation of an oxide- or alcohol-bridged network (the gel) by a polycondensation reaction.
- Aging of the gel, which means that during that period the gel transforms into a solid mass.
- Drying of the gel: in this step, water and other volatile liquids are removed from the gel network.
- Dehydration: the material is heated at temperatures up to \(800\ ^\circ\text{C}\).
The sequence is collected in FC 16.5.
Flowchart
FC 16.5 – Sol-gel synthesis of nanomaterials: metal alkoxide precursor → hydrolysis with water → stable sol of colloidal particles → polycondensation (gelation) → gel with liquid trapped in pores → aging → solid mass → drying (removal of water and volatile liquids) → dehydration by heating up to 800 °C → oxide nanomaterial; branch from the sol: spinning/coating → thin film
16.7.3 Analysis or Characterization of Nanomaterials
Making a nanomaterial is only half the work. Since the particles cannot be seen, weighed out individually or tested by an ordinary chemical test, we must ask instruments to tell us what has actually been formed — whether nanoparticles were produced at all, how big they are, what shape their surface has, and which functional groups are attached to them.
The synthesized material is analyzed by various analytical tools or techniques. The name of each technique and its use are described in Table 16.1.
Table 16.1
| Name of technique | Instrument used | Information |
|---|---|---|
| 1. UV-visible spectroscopy | UV-visible spectrophotometer | Preliminary confirmation of formation of nanoparticles |
| 2. X-ray diffraction (XRD) | X-ray diffractometer | Particle size, crystal structure, geometry |
| 3. Scanning electron microscopy | Scanning electron microscope (SEM) | Structure of surface of material, that is morphology |
| 4. Transmission electron microscopy | Transmission electron microscope (TEM) | Particle size |
| 5. FTIR, Fourier transform infrared spectroscopy | Fourier transform infrared spectrophotometer | Absorption by functional groups, binding nature |
Notice how the techniques divide the work between them. UV-visible spectroscopy gives the first quick indication that nanoparticles have formed, because their colour and absorption depend on size. XRD and TEM both report on size, but XRD does so for the crystal as a whole while TEM looks through an individual particle. SEM shows what the surface looks like, and FTIR reports which functional groups are present and how they are bound.
| Column A (requirement) | Column B (technique) |
|---|---|
| (a) Identify the functional groups attached to the nanoparticle surface | (i) TEM |
| (b) Measure the size of an individual particle in a transmitted-electron image | (ii) FTIR |
| (c) Obtain the crystal structure of the sample | (iii) SEM |
| (d) Study the morphology of the material’s surface | (iv) XRD |
16.7.4 Photographs of Instruments
The instruments named in Table 16.1 are shown in the photographs and schematic drawings that follow, so that the technique can be connected with the apparatus that performs it.
The X-ray diffractometer used for XRD measurements is shown in Fig. 16.8.
Diagram
Fig. 16.8 – Photograph of an X-ray diffractometer, showing the circular goniometer housing with the X-ray source and detector arms mounted on it
The working parts of a scanning electron microscope are drawn in Fig. 16.9, and the complete instrument is photographed in Fig. 16.10.
Diagram
Fig. 16.9 – Schematic diagram of a scanning electron microscope, labelled: electron gun, condenser lens (two), objective lens, sample chamber, sample, X-ray detector, backscatter detector, secondary detector, anode, electron beam, vacuum chamber
Diagram
Fig. 16.10 – Photograph of a complete scanning electron microscope installation with its column, sample chamber and control computer
The transmission electron microscope, which sends the electron beam through the specimen rather than across its surface, is shown with its column labelled in Fig. 16.11.
Diagram
Fig. 16.11 – Photograph of a transmission electron microscope beside a labelled column diagram: filament, Wehnelt, anode, gun, gun deflectors, fixed aperture, condenser lens 1 and 2, condenser stigmator, condenser aperture, beam deflectors, objective stigmator, objective lens upper, specimen holder, objective lens lower, objective aperture, selective area or intermediate aperture, image deflectors, intermediate lens, projector lens 1 and 2, different stigmator, CCD camera, viewing screen, beam axis
The instrument used for FTIR measurements is shown in Fig. 16.12.
Diagram
Fig. 16.12 – Photograph of an FTIR spectrophotometer with its sample compartment and the attached computer display
16.8 History of Nanotechnology
Nanomaterials have been produced and used by humans for hundreds of years. However, the understanding of certain materials as nanostructured materials is relatively recent. Craftsmen obtained their results by trial, by recipe and by long experience, without any idea of what was happening at the scale of atoms.
What changed was not the materials but the ability to look at them. Due to the development of advanced tools — that is, sophisticated instruments — it has been possible to reveal the information at the nanoscale, and only then could the old recipes be explained.
Some of the people whose work shaped this field are collected in Fig. 16.13, including the physicist whose 1959 lecture opened this chapter.
Diagram
Fig. 16.13 – Timeline panel of contributors to nanotechnology with portraits and years: Watson and Crick 1953, Richard Feynman 1959, Von Neumann machines 1966, Eric Drexler 1986, Foresight Institute 1989, Center for Responsible Nanotechnology, National Nanotechnology Initiative 1999
Nanomaterials Used Before They Were Understood
- The beautiful ruby red colour of some ancient glass paintings is due to gold and silver nanoparticles trapped in the glass matrix, as seen in Fig. 16.14.
Diagram
Fig. 16.14 – Photograph of two panels of old stained glass showing deep ruby red areas in the robes and background
The decorative glaze or metallic film known as lustre, found on some medieval pottery, is due to certain spherical metallic nanoparticles.
Carbon black is a nanostructured material that is used in the tyres of cars to increase the life of the tyre (discovered in 1900). Carbon nanotubes are made up of graphite sheets with a nanosized diameter, and they have the highest strength.
Fumed silica, a component of silicone rubber, coatings, sealants and adhesives, is also a nanostructured material.
Classification by Shape
Once instruments could reveal these structures, it became useful to sort them by shape as well as by size. The classes are illustrated in Fig. 16.15.
Diagram
Fig. 16.15 – Classification of nanomaterials in four panels: (a) 0D spheres and clusters, (b) 1D nanofibers, wires and rods, (c) 2D films, plates and networks, (d) 3D nanomaterials shown as a stacked block
To the zero-, one- and two-dimensional classes met earlier, this figure adds a third: 3D nanomaterials, in which nanoscale units such as particles, wires or layers are packed together into a bulk solid. The piece as a whole is large, but its internal structure is still built of nanoscale building blocks, which is what gives such a solid its unusual strength or catalytic behaviour.
16.9 Applications of Nanomaterials
Nanochemistry has already contributed to a number of innovative products in various disciplines, because of the unique physical, chemical, optical, structural and catalytic properties of nanomaterials. A few applications are given below. In each of them, look for which characteristic feature from Section 16.6 is actually being used, because that is what turns a laboratory observation into a product.
Stronger, Lighter and Smarter Surfaces
- Nanoparticles can contribute to stronger, lighter, cleaner and smarter surfaces and systems. They are used in the manufacturing of scratchproof eyeglasses, in transport, in sunscreen, in crack-resistant paints, and so on.
A coating only a few tens of nanometres thick adds almost no weight and does not change how a lens looks, yet it changes how the surface behaves — how hard it is, how it handles water, and what it does with light.
Electronic Devices
- Nanomaterials are used in electronic devices. For example, in Magnetoresistive Random Access Memory (MRAM), which stores information using the magnetic state of extremely small elements rather than as stored electric charge, so the data is not lost when the power is switched off.
Water Purification
- Nanotechnology plays an important role in water purification techniques.
Water contains waterborne pathogens like viruses and bacteria. 1.1 billion people are without access to an improved water supply, and the provision of safe drinking water is currently a high priority.
Recently, cost-effective filter materials coated with silver nanoparticles (AgNps) have emerged as an alternative technology — for example, in household water purifiers. Silver nanoparticles act as a highly effective bacterial disinfectant and remove E. coli from water. The route from raw water to safe water is summarised in FC 16.6.
Flowchart
FC 16.6 – Water purification using a silver-nanoparticle filter: raw water containing bacteria and viruses → pre-filter removes suspended solids and turbidity → passes through porous filter material coated with silver nanoparticles → contact with the nanoparticle surface kills bacteria such as E. coli → treated water collected → safe drinking water
Self-Cleaning Materials
- Lotus is an example of self-cleaning. The lotus plant (Nelumbo nucifera) grows in muddy water, and yet its leaves always appear clean. The plants’ leaves are superhydrophobic.
Nanostructures on the lotus leaves repel water, which carries dirt away as it rolls off. The lotus effect is the basis of self-cleaning windows. The mechanism is shown in Fig. 16.20.
Diagram
Fig. 16.20 – The lotus effect: enlarged cross-section of a lotus leaf surface covered with tiny nanoscale bumps and a waxy coating; a nearly spherical water droplet resting on the tips of the bumps with a large contact angle marked, and arrows showing the droplet rolling off and picking up dirt particles on its way
The reason the droplet picks up the dirt is worth stating plainly. On such a surface a water drop touches only the tips of the nanoscale bumps, so very little of it is in contact with the leaf. A dirt particle sitting on those same tips is held far more weakly by the leaf than it is by the passing drop, so it leaves with the water. On a smooth surface the drop would spread, wet the dirt in place and dry there as a stain.
16.10 Nanoparticles and Nanotechnology
Any technology powerful enough to be useful is powerful enough to cause harm, and nanotechnology is no exception. This section sets the gains against the risks.
Advantages
Revolution in electronics and computing. Smaller components mean more circuitry in the same space, and faster, lighter devices that consume less power.
Energy sector. Nanotechnology will make solar power more economical, and energy storage devices will become more efficient. Both improvements matter for the wider argument of this chapter: cheaper solar electricity and better storage reduce the amount of fuel that has to be burnt for the same service.
Medical field. The manufacture of smart drugs helps cure faster and without side effects. Here “smart” means that the drug is carried to the site where it is needed rather than being spread through the whole body, so a smaller dose does the work and healthy tissue is spared. This approach is being applied to the curing of life-threatening diseases like cancer and diabetes.
Disadvantages
Despite the possibilities and the advancements that nanotechnology offers to the world, there also exist certain potential risks involved with it.
Nanotechnology has raised the standard of living, but at the same time it has increased pollution, which includes air pollution. The pollution caused by nanotechnology is known as nano pollution, and this kind of pollution is very dangerous for living organisms.
Nanoparticles can cause lung damage. Inhaled particulate matter can be deposited throughout the human respiratory tract and then deposit in the lungs. Because the particles are far smaller than ordinary dust, the body’s usual defences — the hairs and mucus of the nose and airways — do not trap them efficiently, so they travel deep into the lungs, as traced in FC 16.7.
Flowchart
FC 16.7 – Route of airborne nanoparticles into the body: nanoparticles released during manufacture, handling or use → suspended in air as very fine particulate matter → inhaled → too small to be trapped by nose and upper airways → deposited throughout the respiratory tract → settle in the lungs → tissue damage and other health effects
The characteristics of nanoparticles that are relevant for health effects are size, chemical composition and shape. Size decides how deep a particle can travel and how much surface it presents; chemical composition decides what it can do once it settles; and shape decides how easily the body can clear it away.
For the same reason, nanopowders are handled in the laboratory with the care given to any fine dust — weighed in an enclosure rather than in the open, kept damp or in suspension where the work allows it, and never allowed to become airborne.
Bringing the Two Halves Together
The chapter began with green chemistry and ends with nanotechnology, and the two are not separate subjects. Nano pollution is a new hazard, and the twelve principles tell us how to respond to a new hazard: not by cleaning it up afterwards, but by designing the process so that particles are contained, by using safer solvents in the synthesis, by choosing routes that run at room temperature, and by designing nanomaterials that degrade into harmless substances at the end of their useful life. A technology this powerful has to be built green from the start.
16.11 Summary
Green chemistry: the idea
- Chemistry has improved the quality of life, but the growth of population and industry has made pollution and the energy crisis major global problems of the 21st century.
- Green chemistry is the use of chemistry for pollution prevention, by the environmentally conscious design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. The concept was coined by Paul T. Anastas.
- Its central shift is from treating waste after it forms to designing processes in which the waste and the hazard never arise.
- Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs; adopting the twelve principles of green chemistry is a way of achieving it.
The twelve principles of green chemistry
- Prevention of waste or by-products — prevent waste rather than treat it; zero waste technology also uses the waste of one system as the raw material of another.
- Atom economy — a measure of how many atoms of the starting materials end up in the useful product.
- Less hazardous chemical synthesis — design routes that are as safe as possible (DDT was replaced by BHC, whose γ-isomer is gammexane or lindane).
- Designing safer chemicals — make products that are less toxic or that need less toxic raw materials (adipic acid made enzymatically from glucose instead of from benzene, in the technology developed by Drath and Frost).
- Use of safer solvents and auxiliaries — prefer water and supercritical \(\mathrm{CO_2}\) to volatile halogenated solvents such as \(\mathrm{CH_2Cl_2}\), \(\mathrm{CHCl_3}\) and \(\mathrm{CCl_4}\).
- Design for energy efficiency — run reactions at room temperature and pressure where possible, using catalysts, micro-organisms, renewable materials and better heating technology such as microwave heating.
- Use of renewable feedstocks — prefer plant-based and biological raw materials to depleting ones such as crude oil.
- Reduce derivatives (minimization of steps) — avoid unnecessary protection and deprotection, which need extra reagents, generate waste and lower the atom economy.
- Use of catalysis — catalysts increase rate and selectivity, and reduce waste, reaction time and energy demand.
- Design for degradation — chemicals and their degradation products should not be toxic, bioaccumulative or environmentally persistent.
- Real-time analysis for pollution prevention — monitor and control processes continuously, before hazardous substances form.
- Safer chemistry for accident prevention — choose substances and physical forms that minimize accidents, explosions, fire and emissions.
Atom economy
- \[\%\ \text{atom economy} = \frac{\text{Formula weight of the desired product}}{\text{Sum of formula weights of all the reactants used in the reaction}} \times 100\]
- For the conversion of butan-1-ol to 1-bromobutane, the atom economy is \(\dfrac{137\ \text{u}}{275\ \text{u}} \times 100 = 49.81\ \%\).
- Yield and atom economy are different measures: a reaction may give a high yield and still waste most of the atoms fed into it.
The role of green chemistry
- The Earth has a natural capacity to deal with waste and pollution; we become unsustainable only when that capacity is exceeded.
- Green chemistry promotes innovative technologies in the design, manufacture and use of chemical products, reduces capital expenditure, prevents pollution and promotes industrial ecology.
- It helps protect stratospheric ozone, which is essential for life on the Earth, and helps control the greenhouse effect, that is, global warming.
Nanoscience and nanochemistry
- Nanoscience is the study of phenomena and the manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a larger scale.
- Nanotechnology is the design, characterization, production and application of structures, devices and systems by controlling shape and size at the nanometer scale.
- The nanometer scale is conventionally 1–100 nm, and \(1\ \text{nm} = 10^{-9}\ \text{m}\). ‘Nano’ comes from the Greek word meaning dwarf.
- A nanomaterial has structural components with at least one dimension in the range 1–100 nm; it is larger than a single atom but smaller than bacteria and cells.
- Nanostructures are classified as zero-dimensional (all three dimensions in the nanoscale, e.g. nanoparticles), one-dimensional (two dimensions in the nanoscale, e.g. nanowires and nanorods) and two-dimensional (one dimension in the nanoscale, e.g. thin films).
- Nanochemistry is the combination of chemistry and nanoscience, dealing with the design and synthesis of nanoscale materials of chosen size, shape, structure and composition, and their organization into functional architectures.
Characteristic features of nanoparticles
- Colour: bulk gold is yellow, but about 100 gold atoms arranged in a cube would appear red.
- Surface area: subdividing a bulk solid keeps the volume the same but greatly increases the collective surface area, giving more reaction sites and greater chemical reactivity.
- Catalytic activity: nanomaterial catalysts are usually heterogeneous, are easily separated and recycled — Pd and Pt nanoparticles in hydrogenation, \(\mathrm{TiO_2}\) and \(\mathrm{ZnO}\) in photocatalysis, and gold nanoparticles as catalysts although bulk gold is unreactive.
- Thermal properties: melting point falls with size — sodium clusters of 1000 atoms melted at 288 K, clusters of 10,000 atoms at 303 K, while bulk sodium melts at 371 K.
- Mechanical properties: copper and palladium clusters of 5–7 nm can be up to 500% harder than the bulk metal.
- Electrical conductivity: a carbon nanotube can behave as a conductor or as a semiconductor.
Synthesis and characterization
- Nanomaterials are made by two approaches: bottom up (assembling atom by atom, molecule by molecule and cluster by cluster, as in colloidal dispersion) and top down (breaking bulk solids into finer and finer pieces).
- In the sol-gel process, a sol is a dispersion of colloidal particles (1–100 nm) in a liquid, and a gel is an interconnected rigid network with submicrometer pores. The process is based on inorganic polymerization, is generally carried out at room temperature, and includes hydrolysis, polycondensation, drying and thermal decomposition.
- \[\mathrm{MOR} + \mathrm{H_2O} \longrightarrow \mathrm{MOH} + \mathrm{ROH} \qquad \text{(hydrolysis)}\]
- \[\mathrm{MOH} + \mathrm{ROM} \longrightarrow \mathrm{M\!-\!O\!-\!M} + \mathrm{ROH} \qquad \text{(condensation)}\]
- Its stages are: stable solution of the alkoxide or solvated metal precursor → gelation → aging → drying of the gel → dehydration by heating up to \(800\ ^\circ\text{C}\).
- Characterization techniques: UV-visible spectroscopy (preliminary confirmation of nanoparticle formation), XRD (particle size, crystal structure, geometry), SEM (surface morphology), TEM (particle size) and FTIR (absorption by functional groups, binding nature).
Flowchart
FC 16.8 – The chapter in one sequence: choose a synthesis route (bottom up or top down; sol-gel for oxides) → characterize the product (UV-visible → XRD → SEM/TEM → FTIR) → apply the nanomaterial (coatings, electronics, water purification, self-cleaning surfaces) → assess the risk (nano pollution, inhalation) → redesign the process against the twelve principles of green chemistry, returning to the first step
History and applications
- Nanomaterials have been used for hundreds of years without being understood: the ruby red colour of ancient glass paintings (gold and silver nanoparticles), the lustre glaze of medieval pottery, carbon black in tyres (discovered in 1900) and fumed silica in silicone rubber, coatings, sealants and adhesives.
- The term ‘nanotechnology’ was defined by Nario Taniguchi in 1974, and the invention of the scanning tunneling microscope in 1980 led to the discovery of fullerenes in 1986 and of carbon nanotubes a few years later.
- Applications include scratchproof eyeglasses, sunscreen, crack-resistant paints, MRAM in electronics, filters coated with silver nanoparticles that remove E. coli from drinking water, and self-cleaning windows based on the lotus effect of the superhydrophobic leaves of Nelumbo nucifera.
Nanoparticles and nanotechnology: gains and risks
- Advantages: a revolution in electronics and computing; more economical solar power and more efficient energy storage; and smart drugs in medicine that cure faster and without side effects, including work against cancer and diabetes.
- Disadvantages: nanotechnology has increased pollution, including air pollution — the pollution caused by it is called nano pollution, which is very dangerous for living organisms.
- Inhaled nanoparticles can be deposited throughout the respiratory tract and settle in the lungs, causing lung damage. The characteristics of nanoparticles relevant for health effects are size, chemical composition and shape.
16.12 Objective Questions
- The development that meets the needs of the present without compromising the ability of future generations to meet their own need is known as
- Continuous development
- Sustainable development
- True development
- Irrational development
- Which of the following is the γ-isomer of BHC?
- DDT
- lindane
- Chloroform
- Chlorobenzene
- The prefix ‘nano’ comes from
- French word meaning billion
- Greek word meaning dwarf
- Spanish word meaning particle
- Latin word meaning invisible
- Which of the following information is given by the FTIR technique?
- Absorption of functional groups
- Particle size
- Confirmation of formation of nanoparticles
- Crystal structure
- The concept of green chemistry was coined by
- Born Haber
- Nario Taniguchi
- Richard Feynman
- Paul T. Anastas
16.13 Check Your Concepts
A factory installs an efficient effluent treatment plant and releases clean water into the river. Its manager claims the factory now follows green chemistry. Do you agree? Justify your answer using the definition of green chemistry.
Distinguish between yield and atom economy. Give one situation in which a reaction has a high yield but poor atom economy.
Explain why an elimination reaction can never have 100% atom economy, whereas an addition reaction can.
DDT was once regarded as one of the most valuable chemicals ever made, and was later withdrawn from general use. Which principle of green chemistry does this history illustrate, and why?
Assertion (A): Protecting a functional group and later deprotecting it lowers the atom economy of a synthesis. Reason (R): The reagents used for protection and deprotection contribute atoms that do not appear in the final product. State whether A and R are true, and whether R is the correct explanation of A.
Water and dichloromethane are both used as solvents. Compare them from the point of view of green chemistry, giving two points of difference.
Why does a process that runs at room temperature usually release less carbon dioxide than one that runs at 500 K, even when carbon dioxide appears nowhere in its equation?
What happens if a chemical is designed to be extremely stable so that the product lasts a long time? Discuss the benefit and the risk, naming the principle involved.
Explain the terms persistent and bioaccumulative, and state why a substance that is both is especially dangerous.
The Earth has a natural capacity to deal with waste. Why, then, does green chemistry insist on preventing waste at the design stage?
A grain of sand and a nanoparticle of the same mineral are both far too small to handle with the fingers. Why is only one of them called a nanomaterial?
Classify the following as zero-, one- or two-dimensional nanostructures and give a reason for each: quantum dot, nanowire, thin film, nanotube.
Why does a metal that is unreactive in bulk, such as gold, become an effective catalyst when prepared as nanoparticles?
Arrange the following in increasing order of melting temperature and justify your order: bulk sodium, a sodium cluster of 1000 atoms, a sodium cluster of 10,000 atoms.
Assertion (A): Nanoparticles of a metal are far more chemically reactive than a lump of the same metal of equal mass. Reason (R): Subdividing a solid increases its total volume, and reactivity depends on volume. State whether A and R are true, and whether R is the correct explanation of A.
Compare the top-down and bottom up approaches to the synthesis of nanomaterials under the headings: starting material, control over particle size, and suitability for large-scale production.
Why must the gel be aged and dried before the dehydration step of the sol-gel process, and not after it?
Match the requirement with the technique.
| Column A | Column B |
|---|---|
| a. crystal structure of the sample | i. XRD |
| b. morphology of the surface | ii. FTIR |
| c. functional groups bound to the particles | iii. SEM |
| d. preliminary confirmation that nanoparticles have formed | iv. UV-visible spectroscopy |
Ancient glassmakers produced ruby red glass using gold. Explain why they are still not described as nanotechnologists.
Explain the lotus effect, and state how it is used in self-cleaning windows.
Why do silver-nanoparticle-coated filters use very little silver and yet disinfect large volumes of water?
“The property that makes nanoparticles useful is the same property that makes them hazardous.” Discuss this statement with reference to catalysis and to lung damage.
Name the three characteristics of nanoparticles that are relevant for health effects, and explain what each one decides.
A new plant is to manufacture nanoparticles. Suggest four changes to its design, each based on a different principle of green chemistry, that would reduce its environmental impact.
Identify the principle of green chemistry applied in each case:
- bottom ash of a thermal power station is supplied to a brick industry;
- sensors continuously monitor a reactor for a toxic intermediate;
- a pesticide is redesigned to break down in soil within weeks;
- adipic acid is made from glucose using an enzyme instead of from benzene.
Write the formula to calculate % atom economy.
Name the γ-isomer of BHC.
Ridhima wants to detect the structure of the surface of materials. Name the technique she has to use.
Which nanomaterial is used for the tyres of a car to increase the life of the tyres?
Name the scientist who discovered the scanning tunneling microscope (STM) in 1980.
Define (i) Green chemistry (ii) sustainable development.
Explain the role of green chemistry.
Give the full form (long form) of the names of the following instruments.
- XRD
- TEM
- STM
- FTIR
- SEM
- Define the following terms:
- Nanoscience
- Nanotechnology
- Nanomaterial
- Nanochemistry
How does nanotechnology play an important role in water purification techniques?
Which nanomaterial is used in sunscreen lotion? Write its use.
How will you illustrate the use of safer solvents and auxiliaries?
Define catalyst. Give two examples.
Explain any three principles of green chemistry.
Explain atom economy with a suitable example.
How will you illustrate the principle of minimization of steps?
What do you mean by sol and gel? Describe the sol-gel method of preparation of nanoparticles.
Which flower is an example of self cleaning?
16.14 Practice With Numericals
(Use atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, S = 32, Ca = 40, Br = 80. Take \(1\ \text{nm} = 10^{-9}\ \text{m}\) and \(1\ \text{Å} = 10^{-10}\ \text{m}\).)
Calculate the percentage atom economy of calcium oxide in the reaction \[\mathrm{CaCO_3}\text{(s)} \xrightarrow{\ \Delta\ } \mathrm{CaO}\text{(s)} + \mathrm{CO_2}\text{(g)}\]
1-bromobutane can also be prepared by the reaction \[\mathrm{CH_3CH_2CH_2CH_2OH} + \mathrm{HBr} \longrightarrow \mathrm{CH_3CH_2CH_2CH_2\!-\!Br} + \mathrm{H_2O}\] Calculate the percentage atom economy for 1-bromobutane by this route, and compare your answer with the value of 49.81% obtained for the route using sodium bromide and sulphuric acid.
Calculate the percentage atom economy of 1,2-dibromoethane in the reaction \[\mathrm{C_2H_4} + \mathrm{Br_2} \longrightarrow \mathrm{C_2H_4Br_2}\] and state what your answer shows about addition reactions.
Ammonia is manufactured by the reaction \[\mathrm{N_2}\text{(g)} + 3\,\mathrm{H_2}\text{(g)} \rightleftharpoons 2\,\mathrm{NH_3}\text{(g)}\] Calculate the percentage atom economy for ammonia.
A cube of a solid of side 1 mm is divided into small cubes of side 100 nm. Calculate (a) the number of small cubes formed and (b) their total surface area.
A cube of a metal of side 1 m is cut into cubes of side 1 µm. Calculate the total surface area of all the small cubes and the factor by which the surface area has increased.
A spherical nanoparticle has a diameter of 20 nm. Express this diameter (a) in metres and (b) in ångström units.
How many nanoparticles of diameter 5 nm, placed in a line and touching one another, are needed to span a length of 1 µm?
The dehydration step of a sol-gel preparation is carried out at 600 °C. Express this temperature in kelvin.
Bulk sodium melts at 371 K, while a cluster of 1000 sodium atoms melts at 288 K. Calculate (a) the lowering of the melting temperature and (b) this lowering as a percentage of the bulk melting temperature.
The hardness of a 5–7 nm palladium cluster is stated to be up to 500% greater than that of the bulk metal. If the hardness of bulk palladium is taken as \(H\), express the hardness of the cluster in terms of \(H\).
A carbon nanotube has a diameter of 2 nm. Calculate how many such nanotubes, laid side by side and touching, would be needed to cover a width of 1 mm.
1 nm = ….. m?