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Crystals are something we come across every day. From the salt on your table to the ice in your cup, from your phone screen to the crystals sold for good luck, it’s unlikely that you can make it through the day without encountering at least a crystal or two. When all’s said and done though, what makes crystal structures so different and special from any other materials, and how do we know all that we do about them?

Just like crystals are part of our everyday life, the study of crystal structure is an integral part of physics and chemistry in identifying the chemical composition of a structure. This however cannot be done by observation alone and instead a technique called x-ray diffraction is used. Crystals are solids that are made up of building blocks such as atoms or molecules. These building blocks come together in a repeating pattern with an ordered arrangement to form a crystal. It is because of this long range order that crystalline solids can be studied using x-ray diffraction. This is due to the fact that the uniform spacing between the atoms and molecules in a crystal have a similar size to the wavelength of the x-rays that are used in x-ray diffraction. Solids that are not crystalline (amorphous solids) cannot be studied using x-ray diffraction as they do not have this uniform spacing.

To understand x-ray diffraction, we must first understand how the periodicity (repetitiveness) of a crystalline solid can be described. Each of the repeating building blocks or motifs in a crystal are called lattice points and as they repeat in the same pattern, i.e. are periodic, they can come together to form something called a lattice, which is used to describe the periodicity of the crystal. Crystals can be viewed as structures made up of “unit cells” which are the smallest group of atoms or molecules that when used with the lattice, make up the entire crystal structure.

The unit cell of a crystal can come in many different shapes and forms such as cubic and tetragonal and within those crystal systems, there are four different arrangement atoms can take, primitive, body centred and end-face centred. In order to define the lattice, the locations of the atoms on the unit cell, the symmetry of the crystal structure and the lattice parameters must be known. The lattice parameters are the lengths of each side of the unit cell and they are related to Miller indices and Miller planes. Miller planes are the set of parallel planes that exist on the axes of unit cells, and can be described by a set of three numbers called the Miller indices of the crystal.

Bragg’s Law, the Principle Used in X-Ray Diffraction (1)

This is where x-ray diffraction comes in, as the Miller indices can be found from the results of this technique. In x-ray diffraction, the x-rays are emitted and passed through a crystal. When this happens, the planes of the crystal reflect some of the x-rays at a scattering angle of theta. This angle theta corresponds to a Miller index. This relationship can be found using Bragg’s law as shown in the image above. X-ray diffraction provides an x-ray diffraction pattern of these different values of theta and so give the different Miller indices of the crystal. These can be used to find the unit cell parameters of the crystal, and give the type of unit cell that the crystal has. This pattern can also be compared to a database of diffraction patterns of known substances to identify the likely chemical composition of the crystalline solid or powder that is being analysed by x-ray diffraction.

With that we see how x-ray diffraction can be used to find out the lattice parameters of the unit cell of a crystal and from this its crystal structure, and can also be used to obtain the chemical composition of an unknown crystalline substance and thus is an incredibly important tool in the study of crystal structures.

(1) Image credit: X-Ray Diffraction, Veqter: https://www.veqter.co.uk/residual-stress-measurement/x-ray-diffraction

Figure 1- showing the Ternary phase diagram for the C, H systems.[1]

Dimond like carbons (DLC) are a distinct set of amorphous carbon materials which share structural similarity to diamond, due to them both possessing sp3 hybridised carbon atom. The irregularity in its arrangement comes from the presence of filler atoms such as sp2 hybridised carbons, metals and even C-H bonds. The presence and arrangement of these filler atoms in the sp3 system, causes the material to exhibit unique and/or improved properties not observed in a simple. In fact there are 7 classified type of DLC commonly used (demonstrated in figure 1), the most abundant of which being tetrahedral amorphous carbon (ta-c), consisting of an even blend of sp3 and sp2 carbons, making the material stronger, smoother, have a higher gas barrier performance as well as a better biocompatibility compared to diamond, some research suggesting that it is even possible to scratch diamond.

 

Why and What are they used for?

The unique properties of DLC materials have made them one of the most sought after and researched materials within a large range of fields. The amazing this about DLCs is that in most applications they are only needed as coating materials, which are most formed as thin films (about 5 m thick), this means that not a lot of material is needed and therefore cuts cost for manufactures. These coats can be used on several materials to improve their hardness, they are so successful at improving toughness, that when steal is coated with DLC and exposed to rough wear, its lifetime was improved from a few weeks to 85 years[1]. Because of this DLCs have been found to have a great importance in the durability of materials ranging from scratch resistant car windows to coating space shuttles to prevent wear during launch, due to their high environmental temperatures. DLCs can be made to have an incredible smooth surface allowing them to have an extremely low friction. This technology has found its way to the locomotive industry, where coated gears and equipment make a perfect replacement to lubricants, as well as increase the longevity of the vehicle.

For a long time now DLCs have been researched and developed for biomedical use due to their superior mechanical properties and biocompatibility, they have already been used in the field of medicine biomedical applications. Implants made from DLCs have shown great success, since tissue can easily adhere to the surface as well as when blood is present a layer of protein is formed around the surface making the body less prone to blood clots and less likely to reject the implant. Because of this, DLC coating has been of great important to the development stents, a device which is able to expand veins and arteries. DLCs, much like diamonds, are very inert, research shows that they are very resistant to acidic substances, making them ideal for storage of highly corrosive and dangerous chemical which would otherwise seep through when using uncoated glass, as well as to protect sensitive equipment[1].

The structure of diamond is well known for being extremely electrically insulating, whilst the its graphite allotrope is very conducting along its planes, since DLCs have an internal structure consisting of a  mixture of diamond and graphite carbons, they have been observed to have conducting properties. The extent of which is directly proportional to the amount of conducting sp2 carbons and doping which the material has, this conductivity is achieved via quantum mechanical tunnelling between sp2 sites (pockets of electron site). Because of this DLCs can be easily manufactured to have a range of different conductivities from super conducting to insulating, this also means that right at a key sp2 percentage semiconducting properties are observed. Moreover, the ease that which DLC’s properties can be modified, means that they can be fine tunned to have a desired band gap for a particular job, making them a very useful and prominent technology in the semiconducting industry[2]. Unfortunately, the market is currently controlled by silicon semiconductors, due to them being cheaper and having more investment, meaning that current DLCs are used to coat and improve on the properties of already developed silicon based semiconductors. Due to the incredible conductivity features that DLCs can manufactured to have, means that they are very regularly used in the electronic community both for passive and active materials.

 

How are they manufactured?

Figure 2- showing 5 types of ion beam deposition methods [2]

Since the invention of DLCs in the early 70s, there have been a multitude of ways to develop them, all of which based on deposition methods to grow thin films. The first DLCs where developed via ion beam deposition, which have expanded into several beam type depositions as shown in figure 2, all of which sharing the general features, carbons ions are created by a plasma sputtering to a graphite surface. Sputtering is the process where an accelerated ion is targeted on to the surface of a material (in this case graphite) to remove particles of the material. The ejected carbon ions can them be guided using a forward bias to a substrate target where the thin films can grow from. Unfortunately, this process does require immense temperatures and a high vacuum environment which reduces the number of materials that it can grow from as many materials might decompose in the process. The other major deposition technique is called chemical vapour decomposition (CVD) where a solid material is vaporised via a chemical reaction and then deposited to the surface of a substrate. This technique is widely used in the formation of thin films for semiconductor materials. [2]

 

Their future?

As mentioned before the properties of DLCs can be highly tuned by controlling their manufacturing process, because of this most current research usually targets the production process of this material. Recent improvements in the field of DCLs focuses on developing deposition methods which do not require a high vacuum and temperature, Keio university in Japan have been studying these novice deposition growths, able to develop thin films at atmospheric temperature and observing any changes in the structure and properties.[3]

 

References:

[1] Rajak, D., Kumar, A., Behera, A. and Menezes, P., 2021. Diamond-Like Carbon (DLC) Coatings: Classification, Properties, and Applications. Applied Sciences, 11(10), p.4445.

[2] American Elements. 2022. Robertson, J., 2002. Diamond-like amorphous carbon. Materials Science and Engineering: R: Reports, 37(4-6), pp.129-281.

[3] Hasebe, T., Ishimaru, T., Kamijo, A., Yoshimoto, Y., Yoshimura, T., Yohena, S., Kodama, H., Hotta, A., Takahashi, K. and Suzuki, T., 2007. Effects of surface roughness on anti-thrombogenicity of diamond-like carbon films. Diamond and Related Materials, 16(4-7), pp.1343-1348.

With the EU’s ambitious goal to be carbon neutral by 2050, it’s increasingly important to examine all aspects of our energy production, storage and usage. At present, buildings account for 40% of the EU’s energy consumption and approximately 75% of the EU building stock is energy inefficient (1). Therefore increasing the energy efficiency of buildings, could be one of the most effective methods of reaching our climate goals.

Although in countries with climates similar to Ireland we mostly focus on retaining heat in buildings, in hotter climates cooling is the most important feature of buildings. Air conditioning is one way to cool these buildings, but it is horribly energy inefficient. With many developing countries having hot or mixed climates, it is predicted that the demand for air conditioning will spiral out of control in the coming decades, unless we can find a better solution. Smart glass, may be this solution.

Smart glass, is a glass whose transmission changes when voltage, light or heat is altered. It can be used in smart windows to change the visible fraction of transmitted light. It thus, minimises the need for cooling and keeps rooms at a comfortable temperature. One of the technologies used for smart windows, are chromic materials. Electrochromic and photochromic smart glass are of particular interest.

Electrochromic smart glass changes its transmittance when it is stimulated by an electrical signal (2). It can change to any state between and including transparent and opaque. An electrochromic glass panel will consist of layers making up a glass stack. This glass stack is usually only a few microns thick. On  the top and bottom a transparent conductive layer (TCO) will be present, while the centre of the structure will contain the electrochromic layer and electrolytic layer. When a voltage is applied to the TCO layer, charged particles move from the electrolyte to the electrochromic layer (2). The electrochromic layer is normally transparent  in its inactive state, but this increase in charge results in an increase in the absorption of light. The reason for this change in transmission is due to the ions from electrolyte inserting themselves into the electrochromic layer.

             Electrochromic smart glass layer (3)

This results in the reduction of the band gap, which means photons with lower energy can now be absorbed by the glass. Electrochromic glass allows the user to control the amount of light by a flick of a switch which means they can regulate temperature or privacy. However due to electrochromic glass’s need for electricity, it is not quite as environmentally favourable as photochromic glass

Photochromic smart glass changes colour with exposure to light (4). It does not need any electricity to work, and is usually manufactured in the form of a thin film on the window. The thin films used are often photochromic rare-earth oxhydrides. As well as the benefit of not needing electrical power, the decrease in transmittance of rare earth thin films extends from the UV up to mid-IR (5). This means that photochromic smart glass can reduce solar thermal gain, which can often overheat a room. This in addition to the glass being able to reduce visible light. The main problem of photochromic smart windows, is their slow switching speeds and their stability over long periods. At present research is underway to try and address these problems.

If the problem of the switching speed could be addressed (at present it can take 5 minutes for the glass to return to a clear state), photochromic glass could have major benefits to the transportation industry. It would reduce glare for drivers and also reduce the need for air conditioning for passenger comfort. Electrochromic glass can also take a time in the order of minutes to change state, which again is a disadvantage for transport applications. However it has been suggested for architectural applications the slow switching speed of electrochromic and photochromic glass is an advantage, as it gives our eyes a chance to adjust to the change in light level (2).

                Boeing electrochromic window (6)

Electrochromic and photochromic glass are already utilised in industry. For example the Boeing 787 Dreamliner uses electrochromic windows to replace window shades in the aero-plane, while photochromic sunglasses are becoming more commonplace in everyday life. I expect that in the coming years smart glass will become even more prevalent, as we shift our attention to climate adaptive structures which can reduce are energy usage.

References

  1. European Comission. In focus: Energy Efficiency in buildings. European Comission website. [Online] February 17, 2020. [Cited: May 11, 2022.] https://ec.europa.eu/info/news/focus-energy-efficiency-buildings-2020-lut-17_en#:~:text=Collectively%2C%20buildings%20in%20the%20EU%20are%20responsible%20for,mainly%20stem%20from%20construction%2C%20usage%2C%20renovation%20and%20demolition..
  2. Smart Glass World. What is electrochromic smart glass? Smart Glass World Web site. [Online] 2022. [Cited: May 13, 2022.] https://www.smartglassworld.net/what-is-electrochromic-glass.
  3. Infinity SAV. Smart Glass Inifinity Sav Web site. [Online] 2022. [Cited: May 13, 2022]
  4. Durr, Heinz and Bouas-Laurent, Henri. Photochromism Molecules and Systems. s.l. : Elsevier Science, 2003. ISBN: 9780080538839.
  5. Colombi, Giorgio. Photochromism and Photoconductivity in Rare-Earth Oxyhydride Thin Films. Delft : TU Delft, 2022.
  6. Coxworth Ben. Electronically-dimmable windows on offer for Boeing 777x. New Atlas Web site. [Online] January 07, 2020. [Cited: May 13, 2022]

 

 

Carbon nanotubes are very tiny lightweight cylinders made up of carbon atoms, usually measuring on the nanometre scale. On the molecular level, carbon nanotubes (often abbreviated to CNT) are 100 times stronger than steel, as well as being 1/6th the weight of steel. Along with these mechanical properties, CNTs are also some of, if not the best conducting materials for heat and electricity. Due to these many properties, these CNTs have been on the minds of many physical and chemical scientists (known as nanoscientists) very recently, as they could help in the creation of many new types of technology, including longer lasting electric car batteries, which would help in reducing climate change.

The first recorded use of carbon nanotubes may surprise some, as they were first used in mediaeval times, although unbeknownst to their creators. Swords from this time have been examined, and some of them have been shown to contain CNTs, with each tube only being slightly larger than half a nanometer in thickness. Marianne Reibold and colleagues from the University of Dresden looked at ‘Damascus steel’ swords, where they found they had a content of around 1.5% carbon. This should have made the swords brittle, however these swords were very malleable and had a terrific hardness, this is why these swords have gone down as something from legends.

In 1952 Scientists L. V. Radushkevich and V. M. Lukyanovich reported clear photographs of 50-nanometer diameter carbon tubes in the Soviet Journal of Physical Chemistry. However, as the study was published in Russian, and Western scientists’ access to Soviet press and studies were restricted at this time, due the Cold War, this finding went virtually ignored. Carbon nanotubes were most certainly created before this date, but the inventions of the transmission electron microscope allowed scientists to finally see what they created.

Then in 1958, the physicist Roger Bacon was examining carbon at a temperature and pressure that would allow the solid, liquid, and gaseous phases of carbon to coexist at the same time, known as the triple point in thermodynamics. However when examining the carbon, he discovered a new carbon fibre that would change the course of nanoscience forever. These carbon fibres were hollow in the middle,being around a few micrometres in width, and almost 3cm in length. While not technically CNTs, these carbon fibres were the first stepping stones towards the creation of CNTs.

However, most scientists are of the consensus that the first proper creation of these CNT were first done by the Japanese scientist Sumio Iijima. Iijima made these CNTs using a method known as the ‘arc discharge evaporation method’. This is where a material, in Iijima’s case carbon, is vaporised between two electrodes. This vapourized material is then condensed out to grow nanoparticulate materials. The CNTs discovered here have become the main format of what CNTs look and act like to this day. Due to this, Suono Iijima is now known as the godfather of modern carbon nanotubes.

The applications of CNTs are endless. One current use of them is for a product known as ‘Gecko tape’. Sold commercially, gecko tape is able to stick and hold lightweight materials to smooth surfaces, almost like double sided tape. It was given its name as it was modelled after how gecko’s feet are able to stick to materials, even just one toe at a time, using van der waals forces. This tape is made of a polymer embedded with synthetic setae, kind of like needles, made of CNTs. These CNTs allow the tape to stick and come off from smooth materials quite easily, leaving no residue. They are even able to work at the most extreme of temperatures.

A potential use for these CNTs that has been heavily looked at is for drug delivery within the body. The nanotube would allow for medicine doses to be reduced by localising delivery, as well as considerably lowering the cost for customers. The medication would be carried by the nanotube in one of two ways: it can be connected to the side or trailed behind the tube, or it can be inserted directly inside the tube. Both of these approaches are effective for medication delivery and distribution inside the body.

Carbon nanotubes have only started to change the world, but already their effects can be seen all over. It is incredible to see how they have evolved, from humble beginnings being used in swords of mediaeval times, to potentially being used as a drug delivery system, it is fair to say that Carbon nanotubes are ‘the’ future of technology.

What is a TCO?

The widely used TCO indium tin oxide (ITO)

The acronym TCO stands for transparent conducting oxide, which as the name suggests, is a metal oxide material which conducts electricity and remains optically transparent. This subset of novel materials combines some of the most desirable attributes of materials such as metals for their relative conductivity, as well the transparency which comes with insulative materials such as glass. TCOs have applications throughout the electronics industry in fully transparent electrical devices such as transparent displays and lighting, and optoelectrical and photovoltaic devices.

How do they work?

TCO materials are semiconductor materials, meaning there is an energy gap (known as a band gap) between the occupied electronic states in the material (known as the valence band) and unoccupied electronic states in the material (known as the conduction band) and have conductivity values between those of a pure metal and an insulator. What separates TCO materials from standard semiconductors is that the optical band gap energy (the difference in energy between the energy levels of allowed optical electron transitions) is greater than the energy of visible light. This means that the optical band gap for the TCO material must be larger than the energy of violet light which has a wavelength of ~ 400 nm. This corresponds to an energy of 3.1 eV. What this means is that a band gap bigger than 3.1 eV will make the material ‘see through’.

Band structure of a TCO[2] highlighting the desirable band gap (Eg,opt), the conduction band minimum (CBM), the valence band minimum (VBM) as well as the desired curvature for a n-type TCO.

In order to ensure we have lots of charge carriers ready to move around and conduct within the material, impurities in the form of different atoms are added into the material which either have additional electrons (n-type doping) or fewer electrons (p-type doping) than the atoms already in the material. In this way, additional charge carriers are added into the unoccupied states in the material, allowing for greater conductivity. This also can increase the transparency of the material, by making the band gap bigger. One can alter the parent material, its structure and the type of extra atoms added to create a whole host of TCO materials.

Some desirable design attributes for TCO materials are:

  1. We want high mobility in the conduction/valence band for charge carriers to move and create current.
  2. We want large numbers of charge carriers within the material to move and create current.
  3. We want to have an optical band gap which is greater in energy than visible light so that the material appears see through. This can be improved by adding different atoms into the material (Moss-Burstein shift).
  4. The gap between the conduction band minimum and the next higher conduction band minimum (known as CBM and CBM + 1) must be greater than 3.1 eV to ensure transparency.

So, what’s the big deal?

As we strive to become ever greener in way we live and consume energy, the need to create novel devices and electronics becomes greater. One major application of both n-type and p-type TCOs is in photovoltaics and in the improvement of solar panels. When light is incident on a solar panel, the individual photovoltaic cells (which are usually silicon-based p-n junctions) generate electron-hole pairs which are able to move around and generate current within the circuit. The theoretical efficiency of a multi-junction solar panel with an infinite number of layers using sunlight as the incident light is 68.7%[3] and 86.8%[4] using focused concentrated light. In reality, this efficiency is a lot lower, with William Shockley and Hans-Joachim Queisser estimating that the maximum efficiency for a single p-n junction solar cell was 33.2%[5] with a band gap of 1.34 eV[5]. Commercially available silicon-based p-n junctions’ solar cells having an efficiency of 24.4%[6]. At the front of the solar cells, TCO layers are used and act as the optically transparent electrode that allows photons into the solar cell and transports the photo-generated electrons to the external device terminals. It has been shown that the use of new TCO materials can be optimised by tuning the refractive index and matching materials to minimise losses due to internal reflections at interfaces, increasing the efficiency of standard solar cells by 15%[7], allowing for greater conversion of light into electricity by the solar cells.

Layout of an AMOLED screen[2] which comprises of many OLED pixels utilising TCO electrodes.

Another use of TCO materials is in optically transparent displays. The TCO material functions as a transparent electrode which can form one of the layers in organic light emissive devices (OLEDs) used in touch panels, flat panel displays and other future devices. The most commonly used TCO for these applications is n-type indium tin oxide (ITO) which features very high conductivity and high levels of transparency, excellent for display electronics. Similar to their application in solar cells, tuning the way the TCO interacts with other layers in the display greatly influences the performance of the devices, improving the efficiency and stability of the displays.

References:

[1] Electan.com. (2022). ITO (Indium Tin Oxide) Coated PET Plastic – 100mm x 200mm. Available at: https://www.electan.com/ito-indium-tin-oxide-coated-pet-plastic-100mm-200mm-p-8272-en.html

[2] Dixon, S.C., Scanlon, D.O., Carmalt, C.J. and Parkin, I.P. (2016). n-Type doped transparent conducting binary oxides: an overview. Journal of Materials Chemistry C, [online] 4(29), pp.6946–6961. doi:10.1039/c6tc01881e.

[3] A. De Vos & H. Pauwels (1981). “On the Thermodynamic Limit of Photovoltaic Energy Conversion”. Appl. Phys25 (2): 119–125. Bibcode:1981ApPhy..25..119Ddoi:10.1007/BF00901283S2CID 119693148.

[4] De Vos, A. (1980). “Detailed balance limit of the efficiency of tandem solar cells”. Journal of Physics D: Applied Physics13 (5): 839–846. Bibcode:1980JPhD…13..839Ddoi:10.1088/0022-3727/13/5/018.

[5] William Shockley and Hans J. Queisser (March 1961). “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells” (PDF). Journal of Applied Physics32 (3): 510–519. Bibcode:1961JAP….32..510Sdoi:10.1063/1.1736034.

[6] G. Watson (2022). Solid State Materials and Modelling: Topic 7 Doped Semiconductors. https://tcd.blackboard.com/bbcswebdav/pid-2127782-dt-content-rid-12902411_1/courses/CHU33307-202122/CHU33307-Solid%20State%20Materials%20and%20Modelling-Topic%207%20Doped%20semiconductors.pdf

[7] K. Fleischer, E. Arca, I.V. Shvets, Improving solar cell efficiency with optically optimised TCO layers, Solar Energy Materials and Solar Cells, Volume 101, 2012, Pages 262-269, ISSN 0927-0248, https://doi.org/10.1016/j.solmat.2012.01.037. (https://www.sciencedirect.com/science/article/pii/S0927024812000530)

Focus on Climate Change

The greatest challenge of the 21st century will surely be the concerted attempt to supersede traditional carbon-based fuels with renewable and cleaner energy sources. In 2021 human activity contributed 36.3 billion tonnes of carbon dioxide into the environment according to the International Energy Agency ([1] IEA, 2022). Most notably carbon dioxide retains excess solar heat in the atmosphere which is driving rapid variations in temperatures and weather patterns across the planet. Other pollutants such as sulphur dioxide and  nitrogen oxides, which can be emitted from car exhausts, contribute to worsening acid rain that weakens higher altitude forests and vegetation. There is now a significant inter-disciplinary effort between physicists and chemists to develop novel technologies that will enable cleaner energy production in the near future.  These green sources must satisfy an ever-increasing global energy demand whilst simultaneously reducing any harmful environmental impacts to meet this challenge.

Figure 1: artist’s impression of terrestrial climate change. Credit: [2] NASA, 2015.

Hydrogen Fuel 

Hydrogen is the prime candidate to be the green energy source of the future. It is the most common element in the cosmos making up 75% of the visible universe ([3] Scharf, 2011). It is an abundant form of renewable energy that only produces water when burned. Most impressive is its stored energy which is approximately three times higher than gasoline per gram. This means that a standard sized car would require 1kg of Hydrogen to drive 100km ([4] Crow, 2019). Hydrogen fuel sounds ideal but it presents difficulties that are inhibiting it from replacing gasoline in our automobiles any time soon. Hydrogen is known for being an explosive and flammable gas. This uncomfortable fact was etched into the minds of the public and regulators after the Hindenburg air disaster in 1937. It is a clear reminder of what can go wrong if hydrogen fuel is not used safely. One other difficulty that has been less publicised is the obstacles that the gaseous state of hydrogen under standard pressures and temperatures present to using it as an everyday fuel source.

Figure 2: Hindenburg disaster in New Jersey on May 6, 1937. Credit: [5] The Guardian, 2017.

The Problem with Gaseous Fuels

Gas molecules possess a great deal of kinetic energy in comparison to tighter held particles in solid and liquid states. It would be a nauseous experience if we could sit and ride along on a gas molecule. The molecule would be careening through space while colliding with its neighbours and ricocheting off the walls of the container. Thence it is unsurprising that gas molecules dislike being close together under standard conditions. In fact a fuel tank would need a volume of 11m3 to store just 1kg of hydrogen under atmospheric pressure at room temperature. As previously mentioned this would just be enough to travel only 100km (from Dublin to Tullamore). Hydrogen would need to be compressed at 700bar to squeeze in 5kg of the element at room temperature. ([4] Crow, 2019). This would allow a hydrogen powered car to travel the length of Ireland which is a reasonable range. However the pressurised fuel tank would have to be reinforced adding weight and hence decreasing the predicted mileage. A more fundamental problem is that regulators stipulate that fuel tanks used in consumer vehicles must be limited to pressures of at most 100bar ([6] Dumé, 2020). Luckily there is one other property of gases that may offer a promising low-pressure storage solution. The property is that gas molecules can be made to stick to surfaces and the solution comes in the form of nanoparticles called metal-organic frameworks.

Introducing MOFs

Metal-organic frameworks (MOFs) are porous crystalline materials that act like nano-sponges. They consist of metal ions that are connected via organic linker molecules and physically look like cages that can trap gas molecules inside them. MOFs are highly porous with an astonishing surface area commonly above 2000m2/g. NU-1501-M is an example of a promising MOF that was  synthesised at Northwestern University in the United States. The M stands for metal which is either aluminium or iron. The nano-material can store 14% of its weight as hydrogen which sails past the 4.5% target set by the US Department of Energy in 2020 ([6] Dumé, 2020). NU-1501-M has enough internal surface area in just 1g to cover 1.3 American football fields. Researchers in the Snurr Group at Northwestern University are also designing low temperature tanks that can contain the same volume of hydrogen at 100bar as 700bar by using temperatures as low as -196ºC ([7] Bowser, 2016).

Figure 3: a cartoon of a metal-organic framework trapping a gas molecule inside its cage-like structure. Credit: [7] Bowser, Chemistry World, 2016.

The properties of these materials are impressive and the idea behind using them is rather rudimentary. The principle being employed here is that hydrogen gas molecules can be made to stick to these huge internal surface areas thus allowing larger volumes of gas to be stored without requiring highly pressurised containers. This provides an opportunity for the production of a hydrogen fuel tank that would satisfy current regulations.

MOF Design and Self-Assembly

The natural question arises as to how physicists can design and build these elegant nano-cages in the lab. The formation of a MOF takes place via a thermodynamic process called self-assembly. In physics, self-assembly occurs when ordered structures spontaneously emerge from disordered building blocks. It is an equilibrium process governed by the laws of thermodynamics. The second law of thermodynamics tells us that a quantity called the free energy (ΔG) must decrease for the process to happen spontaneously. Self-assembly is also responsible for the formation of other complex structures such as the DNA double-helix molecule.

Computational simulations are used to analyse metal-linker combinations. There are too many possible combinations to synthesise all of them and so virtual analysis is required. The Snurr Group at Northwestern has alone tested over 13,000 different MOFs for optimal low temperature storage of hydrogen ([7] Bowser, 2016). The amazing thing about MOFs is the customisability that they offer. It’s rather like a high-tech Lego set on the nanoscale. The pore size and polarisability (i.e. the stickiness) are two important characteristics that can be fine-tuned. The pores should be big enough to fit the particular gas molecule inside and sticky enough to just hold the molecule in place. In fact the design process can go one step further to create a single MOF with multiple pore sizes.

Figure 4: Simulation of a MOF designed to store hydrogen. The yellow points here indicate the binding sites. Credit: [4] Crow, Chemistry World, 2019.

Using MOFs as Emission Filters

Metal-organic frameworks offer yet another solution to curb the presence of deleterious gases in the atmosphere. MOFs can be used as gas filters to remove harmful compounds from the air. The application operates under the same physical principle utilized in the low pressure storage of hydrogen. A copper based MOF called Cu-BTC has recently been designed to carry out this very function. It is being researched as a means of separating and storing multiple gas types e.g., separation of carbon dioxide from methane and carbon monoxide. The application of MOFs as nano-filters could be a powerful tool in achieving a cleaner planet. It is conceivable that MOFs could be placed in our rivers and oceans to remove pollutants molecule by molecule. Further research is still ongoing as to how to process and neutralise the trapped molecules. How can we wring the sponge so to speak without allowing the dirty water back into the environment.

Figure 5: a cartoon of the MOF Cu-BTC containing three cages for storing and hence separating three different kinds of gas molecules. Credit: [8] Grajciar et al, 2011.

A Promising Solution

The current development of metal-organic frameworks specific to low-pressure hydrogen storage looks promising. The area of research is still very much in its infancy but the demand for cleaner fuel sources is rising by the day. Countries such as Japan are leading the way and are aiming  to have 800,000 hydrogen powered vehicles on their roads by 2030 ([4] Crow, 2019). Hydrogen still remains a highly combustible and explosive fuel source. However MOFs may be the key to taming this temperamental element. Its natural abundance, energy density and production of just water when burned in oxygen make it much too tempting to ignore.

References

[1] Press Statement by International Energy Agency  I.E.A. (2022), Global CO2 emissions rebounded to their highest level in history in 2021, Available at: < https://www.iea.org/news/global-co2-emissions-rebounded-to-their-highest-level-in-history-in-2021 > [Accessed: 08/05/2022]

[2] Tenenbaum, LF. (2015). When global warming gets you down, come back stronger, Available at: < https://climate.nasa.gov/ask-nasa-climate/2352/when-global-warming-gets-you-down-come-back-stronger/  > [Accessed: 08/05/2022]

[3] Scharf, CA. (2011). The molecules that made the universe, SCIENTIFIC AMERICAN, Available at: < https://blogs.scientificamerican.com/life-unbounded/httpblogsscientificamericancomlife-unbounded20110802the-molecules-that-made-the-universe/ > [Accessed: 08/05/2022]

[4] Crow, JM. (2019). Hydrogen storage gets real, Chemistry World, Royal Society of Chemistry, Available at: < https://www.chemistryworld.com/features/hydrogen-storage-gets-real/3010794.article > [Accessed: 09/05/2022]

[5] Walters, J. (2017). The Hindenburg disaster, 80 years on: a ‘perfect storm of circumstances’, The Guardian, Available at: < https://www.theguardian.com/us-news/2017/may/07/hindenburg-disaster-80th-anniversary > [Accessed: 08/05/2022]

[6] Dumé, I. (2020).  Ultraporous metal-organic frameworks could make clean energy carriers, physicsworld, Institute of Physics, Available at: <https://physicsworld.com/a/ultraporous-metal-organic-frameworks-could-make-clean-energy-carriers/ > [Accessed: 09/05/2022]

[7] Bowser, S. (2016). MOF menagerie fuels search for hydrogen storage solution, Chemistry World, Royal Society of Chemistry  Available at: < https://www.chemistryworld.com/news/13000-strong-mof-menagerie-fuels-search-for-hydrogen-storage-solution/1017524.article > [Accessed: 09/05/2022]

[8] Grajciar, L.,Wiersum, AD., Llewellyn, PL., et al. (2011). Understanding CO2 Adsorption in CuBTC MOF: Comparing Combined DFT–ab Initio Calculations with Microcalorimetry Experiments, The Journal of Physical Chemistry  115 (36), 17925-17933, DOI: 10.1021/jp206002d