An atomic force microscope can measure the electronegativity of an individual atom

An atomic force microscope can measure the electronegativity of an individual atom

Fig. one. A polarized chemical bond between an oxygen atom on a surface and a silicon atom at the tip of an atomic force microscope probe can be used to measure the electronegativity of an individual atom. Figure from the discussed article in Nature Communications

The electronegativity of atoms and their groups is an important parameter for predicting the physical and chemical properties of substances. Until recently, only the average value of electronegativity was available to chemists, which was obtained in experiments with a huge number of atoms and molecules. Researchers from Japan and the Czech Republic have shown that using an atomic force microscope can measure the electronegativity of an individual atom on the surface of a sample of a substance. On the one hand, their work once again demonstrates the breadth of possibilities of the atomic force microscopy method. On the other hand, the results obtained will make it possible to predict the activity of heterogeneous catalysts with greater accuracy, which is even more important from a scientific point of view.

Electronegativity is the ability of a chemical element to attract electrons from other atoms with which it forms a bond. The concept of electronegativity underlies all chemical and some physical processes without exception, and any chemist, when planning experiments, directly or indirectly uses it. The fact is that any chemical process is nothing more than the transfer of electrons from one atom to another. Obviously, to predict the results of a chemical reaction, you need to know where the electrons will move. Electronegativity helps to predict the features of their movement.

The concept of electronegativity was proposed in 1932 by Linus Pauling. By the way, this outstanding scientist became the Nobel Prize laureate in chemistry in 1954, including for the development of this concept. Pauling defined electronegativity as a quantitative characteristic of the ability of an atom in a molecule to attract electrons from other atoms to itself, and this formulation remains relevant to this day. He, by measuring the energy of chemical bonds in different molecules in the gas phase (see Thermodynamic phase), built the first scale of values ​​of electronegativity. It is still often cited in both school and university textbooks on chemistry.

Later, other types of scales appeared, based, for example, on the ionization energy of an atom (this is the energy that must be expended to detach an electron from an atom; it is also called the first ionization potential). Despite the variety of approaches to the definition of electronegativity, all existing scales are in good agreement with each other at a qualitative level. The most electronegative element is fluorine, that is, it attracts electrons to itself most of all, while cesium, on the contrary, gives them up most easily.

Theoretically, francium should give electrons the easiest way (this is the answer to the corresponding question often given by schoolchildren and first-year students of chemical universities). However, this alkali metal is radioactive and no long-lived isotopes are known. Therefore, it is almost impossible to obtain francium in quantities sufficient for study. Its maximum mass available for research was only 10−7 grams. Even this was not enough to determine electronegativity, so its value for france is not given in reference books.

Be that as it may, despite the good convergence of various scales of electronegativity, its values ​​available in the reference literature can be considered averaged. They do not take into account some subtle features of the difference in the environment of atoms, since they are obtained as a result of the simultaneous study of large numbers of atoms or molecules.

Nowadays, chemists and physicists have got in their hands a tool that can measure the strength of a single chemical bond – an atomic force microscope (AFM). The probe of the atomic force microscope, connected with a flexible plate exhibiting piezoelectric properties – a cantilever, moves in the immediate vicinity of the atoms located on the surface of the sample under study. When the atoms of the probe and the sample surface approach each other, intermolecular interaction or even a chemical bond occurs between them (depending on the distance between the atoms and on their properties). In such cases, the forces of attraction between the atoms make the cantilever vibrate. Due to the piezoelectric properties of the material from which the cantilever is made, its oscillations generate an electric current. By measuring the strength of this current, one can judge the value of the interaction energy between the microscope probe and the surface atom of the sample under study. For more details on the principle of operation of atomic force microscopy, see the review on “Biomolecule” and the article from “Chemistry and Life” “Look at the atoms, touch the molecule.” In addition, below is a video explanation of the ACM operation in English:

Jo Onoda from the University of Tokyo, together with colleagues from other scientific institutions in Japan and the Czech Republic, decided to establish whether it is possible to use the values ​​of the interaction energy between individual atoms, obtained using atomic force microscopy, to determine the polarization of a chemical bond – the degree of displacement of its electron density to one of the atoms, due to the difference in the electronegativity of the bond partners. To test the functionality of this idea, the researchers embedded oxygen atoms into the silicon surface. Then, using atomic force microscopy, the energy of bonds arising between the surface atoms and the silicon atom located on the AFM probe was measured (Fig. 1).

In addition to oxygen atoms exceeding silicon in electronegativity, in the experiments, aluminum atoms were introduced into the surface of the sample under study, donating their electrons to silicon, as well as germanium atoms, which are characterized by approximately the same ability to attract electrons as that of silicon (Fig. 2).

Fig.  2. The probes of an atomic force microscope can differ in structure

Fig. 2. The probes of an atomic force microscope can differ in structure (it is not easy to ensure that the devices are absolutely identical at the atomic level at the moment), therefore the researchers verified the accuracy of the results obtained using a quantum chemical assessment of the binding energy of probes of different shapes (shown in the lower part of the figure) with different atoms and groups of atoms. The linear relationship between the calculated and experimentally determined values ​​of the probe energies with germanium (a), aluminum (b) and silicon in its oxide SiO2 (c) shows the relevance of the bond energies obtained using AFM (to obtain the presented graphs, the calculated values ​​of the bonds arising between the probe of the shape f and Ge, Al and SiO2). Figure from additional materials to the discussed article in Nature Communications

The equation that Linus Pauling used to calculate the energy of the polar covalent bond A – B looks like this:

[ E_{mathrm{A}text{–}mathrm{B}} = frac12left[E_{mathrm{A}text{–}mathrm{A}} + E_{mathrm{B}text{–}mathrm{B}}right] + 23 cdot left ( chi_ mathrm {A} – chi_ mathrm {B} right) ]

In him EA – B – calculated binding energy between different atoms A and B; EA – A and EB – B – energies of non-polar covalent bonds between identical pairs of atoms А – А and В – В; χBUT and χIN – values ​​of electronegativity of atoms A and B, respectively. Transformations of these equations make it possible to determine χ by measuring the value of the energies E

Pauling himself for magnitude E took the energy necessary to break bonds between a large number of atoms in molecules. For example, the strength of the Si – O bond, equal to 461 kJ / mol, means that for a rupture of 6.02 · 1023 chemical bonds silicon-oxygen, you need to spend 461 kJ.

Onoda for E took a slightly different value – the mechanical energy required to break one silicon – element bond (Fig. 3). Despite the difference in measurement approaches, the electronegativity values ​​calculated from the data obtained using AFM for a single bond and the values ​​obtained by measuring the breaking energy of a large number of bonds coincided.

Fig.  3. Scheme for calculating the binding energy

Fig. 3. Communication energy calculation circuit E between the AFM probe and an atom on the surface in the discussed experiment. The bond strength was determined at least on the curve showing the dependence of the probe – atom interaction energy on the distance between the AFM tip and the atom (the distance is indicated along the horizontal axis in angstroms). Figure from the discussed article in Nature Communications

The electronegativity values ​​determined using atomic force microscopy, derived from the bond energies of the silicon atom at the tip of the AFM probe with oxygen, aluminum, or germanium atoms on the sample surface, were in good agreement with the values ​​that were calculated as a result of experiments with molecules in the gas phase back in the thirties. years of the twentieth century by Linus Pauling. This circumstance suggests that at one time Pauling successfully derived a formula characterizing the affinity of atoms to electrons from relatively simple parameters. So we can be calm about the correctness of the predictions of the properties of substances, deduced from the “tabular” values ​​of electronegativity. Onoda’s experiment showed that the “averaged” values ​​of this parameter available since the twentieth century are applicable not only for large arrays of chemical bonds – they also work perfectly at the level of one molecule or even an individual chemical bond.

Meanwhile, electronegativity is not such an inviolable characteristic of an atom as the charge of its nucleus: it can change when the environment of the atom changes. Therefore, for example, with an increase in the oxidation state of any chemical element, its “craving” to fill its shells with electrons increases. The difference in electronegativity should also be observed for atoms of the same element in a solid crystal. The atoms located inside the crystal and those that are on its surface are characterized by different structures of the electron shells and, as a result, will exhibit different tendencies towards the attraction of electrons.

Nevertheless, until now, discussions about the difference between the electronegativity of “surface” and “internal” atoms have been the subject of only theoretical analysis. The existing approaches to measuring the tendency of an atom to attract foreign electrons are based on experiments either in the gas phase or in solution. Since the environment of particles in a gas and liquid is constantly changing and can be considered as an average value, Pauling’s approach and subsequent calculation schemes could not experimentally confirm the dependence of the value of the electronegativity of an atom on its location.

To understand whether an atomic force microscope can determine the effect of the environment of an atom (sometimes almost imperceptible) on its ability to pull off electron density, the researchers obtained a surface on which there were silicon atoms bound to two oxygen atoms and, therefore, having a positive oxidation state. The electronegativity of such silicon atoms, determined using an atomic force microscope, turned out to be greater than that of pure silicon atoms contained on the surface, the oxidation state of which is zero.

The observed difference is due to the fact that with an increase in the oxidation state, the electron density on the atom decreases, and as a result, it more willingly attracts electrons to itself, “trying” to make up for their lack.

It is assumed that, in addition to determining and correcting the values ​​of Pauling electronegativity using atomic force microscopy, which have theoretical significance, the new technique will be useful for studying the activity of heterogeneous catalysts (heterogeneous catalysts differ in phase from the substances, the reaction between which they catalyze; for example, platinum in reactions of hydrogenation of unsaturated hydrocarbons; see Heterogeneous catalysis). Such catalysts work at the expense of surface atoms and contain a large number of active sites. Correlation of the ability of an atom of an active center to transfer foreign electrons to its catalytic activity will make it possible not only to construct a more detailed model of heterogeneous catalysis of a number of processes, but also to increase the productivity of the corresponding catalysts. This can be done, among other things, by introducing into their surface a larger number of active atoms possessing the electronegativity value necessary for the manifestation of activity.

A source: Jo Onoda, Martin Ondráček, Pavel Jelínek & Yoshiaki Sugimoto. Electronegativity determination of individual surface atoms by atomic force microscopy // Nature Communications… 2017. V. 8. P. 151–155. DOI: 10.1038 / ncomms15155.

Arkady Kuramshin

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