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Nanomaterial production

How nanomaterials are made and the challenges that prevent large-scale adoption of nanotech

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If Earth were the size of a basketball

The circumference of the planet we inhabit is 40,075 km.

Now imagine taking this massive planet, all its lands and oceans, trees and animals, and shrinking it down to the size of a basketball.

If Earth were the size of a basketball, one nanometer would be the size of a marble.

The double helix of DNA is 2.5 nanometers long. Hemoglobin (the protein that carries oxygen throughout the blood) is 5.5 nanometers in diameter. A human hair is 80,000 nanometers wide. One sheet of paper is 100,000 nanometers thick.

The nanoscale is incredibly small and equally as exciting.

Let’s back up a bit…what exactly are nanomaterials?

Nanomaterials are just a bit larger than subatomic materials. Atoms are the building blocks of nanomaterials, with only 80 atoms making up one nanoparticle. These atoms arrange themselves into spheres, geometric shapes, tubes, fibers, or wires, creating materials we know as nanoparticles.

The loose definition of a nanomaterial is:

A material that has at least one dimension between 1 and 100 nanometers (nm).

Here’s a graphic to illustrate just how tiny the nanoscale is ⬇️

Nanomaterials are SMALLER than viruses and DNA. Only a few atoms larger than a single atom itself.

Over millennia, nature has perfected the art of biology at the nanoscale. Many of the inner workings of a cell naturally occur in the nanoscale (cells themself are not nanoscale, but the components that makeup cells are), like the nucleus, cytoskeleton, ribosomes, DNA, RNA, viruses, proteins, and molecules. These are called organic nanomaterials since they are found naturally in nature and biology.

Over the past few decades, humans have started to model nanomaterials found in nature by creating nanomaterials in the lab, called inorganic nanomaterials. Human-made nanomaterials are usually metal-based, like gold nanoparticles, quantum dots, and graphene. Humans can engineer nanomaterials in a bunch of different ways (with different surface layers, different cores, and different kinds of metals) to achieve any outcome imaginable (particular melting point, reactivity, size, etc.) [1]

Nanomaterials can have one, two, or three dimensions in the nanoscale range, which adds to the capabilities of the nanomaterial (aka the more dimensions in the nanoscale the more rules of physics the material breaks). [2]

  1. Zero dimensional [0-D] — all dimensions (x, y, z) are measured within the nanoscale (no dimensions larger than 100nm). Ex: quantum dots, fullerenes, gold nanoparticle.
  2. One dimensional [1-D] —two dimensions (x, y) are smaller than 100nm and the other is outside the nanoscale. Ex: carbon nanotube, nanowires, nanobars.
  3. Two dimensional [2-D] —one dimension (x) is at the nanoscale while the others are outside the nanoscale range. Ex: graphene, carbon-coated nanoplates, nanofilms.
  4. Three dimensional [3-D] — these nanomaterials are not confined to the nanoscale. They have three dimensions (x, y, z) above 100nm but below 1000nm. 3-D nanomaterials are made of 0-D, 1-D, and 2-D elements closely fitted together. Ex: graphite, polycrystals, liposome, dendrimer. [3] [4] [5]
source

What makes nanomaterials so special?

It’s simple:

Nanoparticles have a mind of their own. They like to party💃🎉🪅

Nanoparticles are so interesting to scientists and physicists because they behave much differently than regular particles. In the nanoscale, particles break all the rules of classical physics and throw their own party.

When solid materials (that can be observed by the human eye) are observed under a microscope, not much changes — the particles possess the same properties. But when that same particle is observed under a nanoscope (special microscope for observing particles in the nanoscale), the particle’s properties change drastically from the properties at the larger scale.

At the nanoscale particles can:

  • Be in two states at once
  • Tunnel through solid objects (think Harry Potter going through platform 9 3/4!)
  • Become physically entangled with a particle millions of miles away

They have different melting and freezing points, chemical reactivity, fluorescence, electrical conductivity, magnetic permeability, and even different colors.

Scientists call these crazy-cool properties of particles in the nanoscale possess quantum phenomena.

If you’re interested in learning more about quantum phenomena and what happens at the nanoscale, I wrote an article on quantum physics which you can read here.

The coolest and most useful property of nanomaterials is not on this list:

As the size of nanomaterials decreases, their surface area increases

Gold nanoparticle observed under a nanoscope source

As nanoparticles get smaller, their surface area grows exponentially larger.

As a particle decreases in size, more atoms are found at the surface of the particle compared to the inside of the particle. For example, a 30 nm particle has 5% of its atoms on its surface, at 10 nm, 20% of its atoms are at the surface, and at 3 nm, 50% of its atoms cluster to the surface of the particle.

Because of this, nanoparticles have a much greater surface area per unit mass than larger particles. [6] [7] [8]

Imagine a Canadian quarter. It has a surface area of roughly 3000 square millimeters. If this coin was divided into particles 10nm in diameter, the total surface area of the particles would be 7000 square meters — which is equal to the size of an entire soccer field!

This high surface area to volume ratio makes nanomaterials:

  • Phenomenal catalysts.
  • Great chemical reactants.

This right here (the large surface area of nanoparticles) is the thing that makes nanoparticles so special. All the hype around nanotech is because of their high surface area-to-volume ratio. It allows scientists to engineer nanoparticles to dissolve at certain rates, react with certain materials, and have certain melting and freezing points. [9]

This can be applied to solve some of the biggest problems in the world. Like batteries which have a limited amount of storage — nanoparticles could be engineered to react with lithium in lithium-ion batteries to soak up, and release the charge of ions quicker. This could drastically increase the storage space of batteries, making them much more efficient. [10]

This high surface area means that nanoparticles spark faster reactions, making them ideal for water purification, drug delivery, and fertilizer enhancement.

So ya, nanoparticles are pretty cool. 😎

The big elephant in the room 🐘

Here comes the big unanswered question: how are nanomaterials made?

Do you break down larger materials? (like when carving piece of wood). Or do you put smaller materials together? (like when doing a jigsaw puzzle).

The answer is both! You can use either method to get to the nanorealm. The fancy scientific term for these two approaches is the top-down or bottom-up synthesis method.

Diagram of the top-down and bottom-up nanomaterial production methods source

Top-down

The goal of the top-down method is to break bulk materials down into nanosize materials. Bulk materials start at the size of the centimeter, millimeter, or micrometer range, and the material is physically broken down until the size of the material is in the nano range.

The problem with this method is that it is very wasteful, since the leftover bulk material can not be further broken down and is discarded. This process also produces nanoparticles of unequal sizes that are not smooth, and have structural defects on the surface (like holes, dents, cracks, and roughness). This significantly impacts the physical and chemical properties of nanoparticles, making them less efficient and reactive. [11][12]

A drawing of a group of nanoparticles synthesized with the top-down method. There are large holes in between particles, a crack down the middle, roughness around the edges, and different densities of particles. source

Bottom-up

In the bottom-up method, chemical reactions are placed on atoms or molecules to create larger clusters of molecules and molecules (nanoparticles). Atoms are forced together by chemical reactions to form clusters, which are then placed under extreme heat to make nanoparticles. The bottom-up method is referred to as ‘wet’ method, since it involves large amounts of liquids, solvents, and chemicals which the atoms are submerged in.

Individual atoms pile up one after another onto a substrate (like a piece of metal) to form molecules, and keep growing until they reach the desired nano size.

Compared to the top-down approach, there is much less waste created, making it more economical and cost-effective. The only problem with this method is scalability — it works very well in the lab, but hasn’t been adopted on a commercial scale yet. [13] [14]

Different methods used to create nanomaterials

There are hundreds of different synthesis methods for creating nanomaterials. In this section we are going to explore four different production methods of nanomaterials, what they are, how they work, and the pros and cons for each method.

Mechanical milling

Top-down method

Mechanical milling (MM) is the simplest and most popular nanoparticle synthesis method. The goal of MM is to reduce the particle size of a bulk material to the nanoscale by grinding particles down with sheer force. It is one of the only synthesis methods where no chemical reaction takes place, and materials are broken down physically instead of chemically.

Pieces of bulk material (like silver or silicon) are placed in small chambers, almost completely filled with steel balls called ‘tumbler balls’. The chamber has horizontal arms called ‘impellers’ which spin horizontally along the bottom of the chamber. The tumbler balls and impellers are what grind the particles down to the nanoscale — the chambers are put on a spinning table and spun around in circles. This causes the metallic balls to roll up and down the side of the chamber, crushing the particles. At the same time, the impellers start to spin, rotating the tumbler balls and nanoparticles around the chamber, ensuring that all the nanoparticles are crushed evenly.

Diagram of the mechanical milling process source

The final products of MM usually have a lot of variability in size and shape and have defects like dents to the surface of the nanoparticle. This is why the nanoparticles created from mechanical milling are usually ground into a powder and utilized as a nanopowder instead of nanoparticles. [15] [16]

Lithography

Top-down method

Nanosphere lithography is another simple and inexpensive synthesis method for creating hexagonal nanoparticles. There are four steps separated into 2 stages: making the mask, and nanostructure production:

  1. Coating: the process starts by coating the starting material with a substrate (like silicon oxide, or polystyrene beads). Common starting materials are aluminum plates, silicon wafers, and zinc stones.
  2. Mask: a nano-thin silica layer called a ‘mask’ is placed over the coated metal.
  3. Decomposition: now a chemical reaction is applied to the base material. The base material is sprayed with a combination of polystyrene beads (which react with the metal to create nanosize pores) and a kind of noble gas (which causes the nanopores to surface to the top of the metal).
  4. Removal: the final step of the process is to remove the mask from the base material. During the chemical reaction, the nanopores surfaced and attached themself to mask, so that then when the mask is taken off, a thin layer of nanowires will come off with it. Think of it like ripping a band-aid off — some of your hair peels off as well.

The mask is then left to settle so that the nanoparticles can manually be harvested from the mask.

This method produces evenly sized hexagonal nanoparticles, with very little structural damage (the mask protects the nanoparticles from structural defects.) The problem with this method is that the harvesting process is very time-consuming, it takes a long time to manually harvest the nanoparticles from the lithography mask. Because of this, lithography is not a scalable process and has not been adopted for commercial use. [17] [18] [19]

Inert gas condensation

Top-down method

Inert gas condensation is a nanoparticle synthesis method used to create metallic and metal oxide nanoparticles.

With inert gas condensation, a material is heated to temperatures higher than its melting point. The material evaporates, forming a vapor.

Any material can be used in inert gas condensation that forms a vapor. Ex: intermetallic compounds, ceramics metals, alloys, semiconductors, and composites. Any metal can be used (Cu, Au, Pd, Ni, Fe).

Image of the chamber used in inert gas condensation source

As the material is evaporating, a noble gas is added to the chamber (noble gases are used instead of other gases because even at high temperatures they do not react. Helium is commonly used). The vapor material collides with the helium gas, causing the vapor to lose its kinetic energy which breaks apart the large vapor molecules into tiny nanoparticles.

The newly created nanoparticles are absorbed by a pipe in the middle of the chamber and pushed through a funnel, where high pressures are applied to the nanoparticles to produce desired size and shapes.

Diagram of the inert gas condensation chamber source

The end product of this process is equiaxed crystallites — a powder that is a few nanometers long. The exact size of the nanoparticle depends on the pressure applied to the vapor — as the base material spends more time in high pressure, the atoms group together to form larger masses of molecules, increasing the size of nanoparticle. Higher pressure equals larger particles, lower pressure equals smaller particles.

Equiaxed crystallites synthesized by inert gas condensation under a microscope Source

The largest drawback is that it is not a scalable system. The mechanics of the process doesn’t work when synthesizing large quantities. [20] [21] [22]

Aerosol synthesis

Top-down method

The aerosol synthesis method is one of the only methods that has been adopted on a commercial scale because of its cheap price, simplicity, and scalable processes. Little material waste is created, and the method allows for high control over nanoparticle size and shape.

Aerosol synthesis method diagram source

The process starts by loading the chamber with a metal. Aerosol is flooded into the chamber, causing the metal to evaporate into droplets 1–10 µm in diameter. Heat by a live flame is applied to the evaporated droplets, increasing overall temperature of the droplets. When the heat is lowered, the droplets crystallize to form a solid nanocomposite.

The last step is for a small mist of water to be sprayed over the droplets — different kinds of nozzles (like hydraulic pressure, pneumatic, ultrasonic nozzle, or rotary disk atomizers) can be used to create small droplets of different sizes. The water causes a reaction in the droplets, causing them to split apart into nanoparticles less than 50nm.

The nanoparticles can then be collected from the inside of the chamber.

The image below shows nanoparticles in the 50nm range created by aerosol synthesis. The far left group of nanoparticles was heated on low heat, which creates less intricate crystal design. The nanoparticles on the far right were heated to a high heat, forming a complex crystal surface. The more complex the crystal structure of the nanoparticle, the more reactive it is. It is this complex crystal structure that makes nanoparticles synthesized by aerosol method great catalysts and reactants.

Nanoparticles created via the aerosol synthesis method. Effect heat has on the crystal structure of nanoparticle. source

Aerosol synthesis:

  • Allows for extreme control over the size and shape of nanoparticles
  • Produces very little to no waste
  • Creates perfectly shaped nanoparticles with a complex crystal shape (making them more reactive than other nanoparticles)
  • Doesn’t use large quantities of solvents and liquids, making it inexpensive and cheap
  • Uses simple materials and solvents making the method very cheap [23] [24] [25]

There’s one ‘but’:

All of these processes work really well in the lab.

The solvents are mixed together, the materials are crushed down to the nanoscale, and nanoparticles are created.

The issue comes with scalability. Transferring the technologies from the lab to the commercial scale has still not happened. Large-scale nanoparticle production is way behind the advances in the lab, leading to high prices of nanoparticles, which leads to reluctancy of adopting the technology.

In this section, we’re going to take a deeper look at the challenges of nanoparticle production.

“Nanomaterials are the trickiest to scale up because of a frequent degradation in performances and batch-to-batch variability.” source

Controlling size and shape

It is very hard to control the size and shape of nanoparticles — most nanoparticles have a wide variety of sizes and shapes, and structural defects on the surface of the particle making them less efficient and reactive. An increase of 10% more surface defects than average can decrease reactivity by 40%. In other words, it is essential that the surface of nanomaterials contains no structural defects, or the reactivity of the material will decrease significantly. [26]

This is a HUGE limitation to large-scale nanomaterial manufacturing.

Since top-down methods involve physically crushing a material into tiny nano-size particles, the particle gets physically damaged. Scratches, dents, cracks, and holes are very common on the surface of nanoparticles, decreasing reactivity.

Pictures from section A show the ideal size, surface structure, and shape of gold nanoparticles. The three pictures from section B show the reality of nanoparticle synthesis: the particles are noncircular with varying sizes and dimensions, and the fuzziness around the edges depicts dents and structural defects. This is the reality of nanoparticle production — the particles are far from perfect. source

Size is another crucial element of nanomaterials. The optimal size for nanomaterials is below 100nm. Most nanoparticle synthesis methods can not reliably produce nanomaterials at the scale. The average size of nanomaterials created by aerosol synthesis is 800nm, small enough that it is still in the nanoscale range, but too large to hold the same properties that nanomaterials in the 100nm range possess. [27] [28]

Price

One gram of gold nanoparticles costs $80 000. While a gram of pure, raw gold goes for $50. [29]

It’s not the gold that makes them expensive, it’s the process of making the gold nanoparticles that is the culprit of the price.

Why? Well, nanomanufacturing involves:

  • Expensive solvents used to suspend the base material in
  • Extremely complex machines to synthesis nanomaterials
  • A lot of energy (expensive) to get very hot temperatures

Put simply: producing nanomaterials is not an easy process. Its complexity is the reason for the high prices.

The other issue with nanomaterial production is it can not be easily automated. At this point, all production methods involve some kind of human interaction; whether it’s mixing solvents, measuring materials, harvesting nanoparticles, or controlling the flow and pressure of gases. All the production methods explored in this article require trained professionals to operate. This makes the manufacturing process of nanoparticles an estimated 4X more expensive. [30] [31] [32]

These two things, controlling size and price, are a massive barrier to large-scale nanoparticle adoption and use.

Nanotech has some very exciting applications in all different fields, from medicine to agriculture, to energy storage.

But nanotechnology’s full potential will not be discovered until cheap and efficient nanomaterial production is created. We need a way to cheaply produce nanomaterials with high control over size and shape.

This is the key to unleashing the true potential of nanotech.

🔬🧪🚀

I’m Rachel, a 16-year-old biotech lover, ultra runner, and podcaster. Check out my other work and connect with me:

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Rachel Lee

Rachel Lee

146 Followers

I’m a 16 y/o biotech lover, podcaster and ultra runner using biotech to improve agricultural productivity in Africa.