Borophene: New focus of research on this most exotic and promising element

Although it was not synthesized until 2015, the origin of borophene dates back to the 90s of the last century. More than two decades ago a group of physicists predicted its existence using computer simulations that described how boron atoms could bond together to form a very thin layer of material only one atom thick.

At that time, technology did not allow the manufacture of a material with these characteristics, but for a few years it has been possible. Borophene is ready, and the expectations of the research groups that are working with these crystals are in the air because, apparently, it has endless applications in fields as attractive as superconductivity or the manufacture of batteries, among others.. Yes, this all sounds like graphene, but it’s worth giving you a chance to get to know it better. This is what borophene promises us.

What is borophene and what makes it so special

The chemical element that we need to produce borophene is, as we can guess from its name, boron. The latter is a semiconductor, which means that depending on the pressure, temperature, radiation or other conditions to which we expose it, it will behave as a conductor of electric current or as an insulator. And, in addition, it is a semi-metal, so it has both some of the characteristic properties of metals and others of non-metals.

Boron is relatively scarce in the earth’s crust. We can find it in rocks such as borax or colemanite, which are formed naturally due to the evaporation of salt-rich water from some high-temperature lakes located in desert areas. We can also find it dissolved in seawater due to the precipitation of boron particles suspended in the atmosphere, as well as the erosion of the rocks that contain it and its circulation through the hydrological cycle, which explains how dissolved boron in water it is transported to the oceans by runoff.

To make a borophene sheet it is necessary to link the boron atoms so that they form a single layer with a thickness of a single atom

The most curious thing is that to make a borophene sheet it is necessary to make the boron atoms adopt a two-dimensional monolayer structure. This simply means that they need to be bonded together so that they form a single layer of boron atoms one atom thick. And achieving it is not easy. In fact, this difficulty largely explains the time that has passed since borophene was discovered thanks to computer simulations until scientists have managed to manufacture it in their laboratories.

How have they done it? Using the same procedure that is used, for example, to produce synthetic diamond: chemical vapor deposition. We do not need to go into complicated details, but it is interesting that we know that this process consists of getting a very hot gas containing boron atoms to condense on a very homogeneous surface of pure silver. The latter is at a temperature much lower than that of the gas in order for the boron to crystallize on it, adopting the form of a single and very thin layer of atoms. We already have our borophene, but… why is pure silver used?

The choice of this precious metal is not the result of chance, as we can imagine. Silver atoms acquire a very uniform crystalline structure and are capable of forcing boron atoms to adopt a very similar configuration. In this way, each boron atom is bonded with a maximum of six boron atoms, giving rise to the formation of a flat hexagonal grid-shaped structure.

chemical vapor deposition chamber
The device in this image is a chemical vapor deposition chamber. Violet plasma is a very hot gas that contains the particles that are going to be deposited to consolidate the material being manufactured.

However, this thin sheet of boron atoms is not completely regular because some of these atoms do not establish bonds with six other atoms of this element, but only with four or five. And this causes the appearance of holes in the structure that are not only not harmful, but are, in fact, responsible for the peculiar physicochemical properties that borophene has.

Two of the characteristics that explain why graphene has generated so much expectation are its extreme hardness and high flexibility. For this reason it is surprising that the scientists involved in the analysis of borophene have confirmed that this material is even more flexible and harder than graphene, which, in turn, has a higher hardness than diamond. The research group responsible for one of the most exhaustive reports devoted to borophene of those that have been published so far is made up of physicists from the universities of Xiamen (China), Singapore and Malaysia, and in it they explain that this material does not stand out only for its extreme hardness and enormous flexibility.

In addition, according to these scientists, borophene is an excellent conductor of electricity, it has a high heat conductivity index (this parameter measures its ability to transport energy in the form of heat), it is very light, it behaves As a superconductor, it has a great capacity for capturing hydrogen atoms and is capable of acting as a reagent, which is why, in theory, it can be used in many chemical reactions. As you can see, the string of interesting properties borophene has is quite impressive.

These are the targeted applications of borophene

The physicochemical properties of this material can be manipulated by acting on the orientation and distribution of the holes in the monolayer structure of boron atoms. For this reason, chemists and materials engineers are likely to find a way in the future to develop different crystal structures of boron atoms with different properties, and therefore with different applications. In fact, some of these borophene variants have already been obtained under laboratory conditions.

According to the physicists who signed the work dedicated to borophene that I have told you about a few paragraphs above, its lightness, high conductivity and high ion transport capacity place it as an ideal candidate to manufacture the electrodes of lithium ion batteries, sodium, potassium, magnesium or aluminum that we currently use. A brief note before proceeding further: an ion is an atom or molecule that is not electrically neutral and therefore has a positive or negative electrical charge.

Borophene is harder and more flexible than graphene, so its possible applications are very interesting

On the other hand, to understand a little better the possible role of borophene in batteries, it is useful to review what an electrode is. The cathode and anode are the electrodes of batteries, and this simply means that they are electrical conductors that are in contact with a non-metallic element in a circuit. In the case of batteries, this non-metallic element is the electrolyte, which we can define as a substance that contains ions, and which, for this reason, acts as an electrical conductor.

The release of electrical energy occurs thanks to a phenomenon known as the redox reaction (reduction-oxidation), which is a chemical process in which a set of electrons travels from one element to another, altering its oxidation state. In our batteries, the cathode is the element that undergoes the reduction reaction, and therefore receives electrons and reduces their oxidation. And the anode is the electrode that does the opposite, that is, it loses electrons, and, for this reason, its oxidation increases.

structure of three experimental borophene crystals
This image shows the structure of three experimental borophene crystals that have a different distribution of boron atoms and their holes.

Another property of borophene that these Asian scientists talk about in their very interesting report is its high hydrogen storage capacity, which in the future could play a fundamental role in the development of new fuel cells, among other possible applications. They also highlight the possibility of using the thin sheets of borophene in the manufacture of supercapacitors with a very high energy density and great stability as long as contact with air is prevented to avoid oxidation of the material.

The distribution of the gaps that remain between the boron atoms of borophen is largely responsible for this material having the ability to behave as a superconductor

Furthermore, the physicochemical properties of borophene vary in the presence of certain molecules, a characteristic that can be exploited to manufacture gas detectors. Its electrical conductivity, for example, is significantly increased in the presence of formaldehyde molecules, which is a highly flammable volatile chemical compound, and therefore also very dangerous. In this context, borophene can be used to detect not only the presence of molecules of this gas, but also of other potentially dangerous chemical compounds, such as ethanol or hydrocyanic acid.

On the other hand, as I anticipated a few paragraphs above, the peculiar crystalline structure that boron atoms make up gives borophene the ability to transform into a superconductor. Precisely, the gaps that remain between some of these atoms due to the fact that a part of them does not establish bonds with six other boron atoms are largely responsible for the superconductivity of borophene being feasible. Thanks to this property, it is likely that this material could be used to make the superconducting magnets that we use, for example, in nuclear magnetic resonance imaging machines in hospitals, in trains that move by magnetic levitation and in particle accelerators.

And, of course, we cannot ignore the possibilities that borophene’s mechanical properties bring to the table, especially its hardness, lightness and flexibility. Thanks to them, it can be used as an alternative to graphene by some of the industries that had focused on the latter material. It is likely that, if scientists can overcome the two challenges that we are going to talk about in the next section of the article, borophene can be used in the manufacture of armor and chassis for all types of vehicles, such as cars, airplanes or ships.

Boron crystals
Boron crystals like the ones in this photograph are the raw material used to produce boron by chemical vapor deposition.

These are the challenges borophene faces to not be just a promise

Throughout this article we have seen that scientists know in great detail the properties of borophene, and this has caused its possible applications to begin to appear on the horizon. However, for this material to have a direct impact on our lives, physicists, chemists and materials engineers will have to solve two challenges that are currently daunting in scope.

Scientists face the challenge of producing borophene on a large scale and preventing its oxidation

One of those challenges, perhaps the most relevant, is finding a way to manufacture borophene on a large scale. Chemical vapor deposition, which is the method currently being used, is a well-known procedure, but it does not seem the ideal solution to industrially produce the amount of borophene necessary for some of the applications that we have discussed in this article. possible.

The other challenge is spurred by the ease with which borophene oxidizes when it comes into contact with air. This characteristic makes it necessary to protect it effectively to prevent it from degrading, which together with the complexity of manufacturing it and the difficulty of producing it in large quantities will make it very expensive. For the moment, we can only wait and trust that scientists can solve these problems. When they do, if they finally succeed, we will continue to tell you.

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