RU2601208C2 - Method of demonstrating self-isolation of metal and splitting of its surface charge - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used for studying surface phenomena. Electrode of investigated metal is brought into contact with ionic liquid. Estance (derivative of surface tension of solid body surface charge density) and current as function of electrode are potential registered. Estance stages are found in form of extremum and bends in range of potentials, where current dependence of potential has no stages. Inclination of estimated dependence of estance-potential between estance steps is measured. Superiority of absolute value of that inclination above one is shown, from which conclusion is made of metal having insulating coating, electrostriction of which takes part in surface tension of solid body. Absence of current changes, corresponding to estance stages, is explained by splitting metal surface charge by insulating coating into inner and outer parts with mutually compensating stages, wherein only inner, stepwise changing part causes electrostriction in insulating coating, generating estance stages, when current is caused by change of total charge without steps.

EFFECT: invention enables to experimentally show connection between spontaneous formation of insulating coating on metal free from oxide, and splitting metal surface charge into two parts, separated by said insulating coating.

11 cl, 10 dwg

Description

The invention relates to visual aids and can be used in the study of surface phenomena.

There is a known possibility of a metal-insulator transition with a gradual increase in the distance between atoms in a metal volume, which contributes to the localization of conduction electrons on these atoms with a sharp increase in resistivity (Mott NF (1967). The transition from metal to insulator. Endeavor Vol. 26, p. 155. Zayman J. (1974) Principles of the theory of a solid body, published by Mir, Moscow, p. 195).

J. (2010). Fundamentals of the physics of solids. Vol. 3. Springer-Verlag, Berlin Heidelberg, p. 476).A known method of demonstrating a metal-insulator transition, including a gradual change in pressure on a sample of oxide V ₂ O ₃ with an admixture of Cr atoms, measuring the conductivity of the sample and registering a pressure threshold corresponding to a jump in electrical conductivity (

J. (2010). Fundamentals of the physics of solids. Vol. 3. Springer-Verlag, Berlin Heidelberg, p. 476).

There is a known method for recording variable surface tension of solids, first proposed by the author (Aut. St. USSR 178161, IPC G01N 13/00, 01/08/1966, Bull. No. 2; Authentic USSR 277399 IPC G01N 13/02, 07/22/1970 , Bull. No. 24; Electrochemistry 1966, volume 2, p. 1061; Reports of the USSR Academy of Sciences 1969. volume 187, p. 601; Electrochimica Acta, 1970, vol. 15, p. 219; book “Surface tension of solids and adsorption” Moscow, Nauka publishing house, 1976, 400 pp .; Uspekhi Fizicheskikh Nauk, 2000, Volume 170, p. 779; J. Solid State Electrochemistry, 2013, vol. 17, p. 1743; RF Patent 2552116, IPC G01N 13/00, 06/10/2015, Bull. No. 16).

This method allows you to measure the derivative of the surface tension of an electrically conductive solid by surface charge density. This derivative is called estance, which is accepted in international literature. In this method, called the estance method, the studied electrically conductive solid in the form of an electrode is fastened with a piezoelectric element and brought into contact with an ionic liquid, for example, with an aqueous electrolyte solution. An alternating current of a given frequency and fixed amplitude is passed through the boundary of the test electrode with the solution. Fluctuations in the surface charge density of the electrode under study are accompanied by fluctuations in its surface tension, which swing the electrode-piezoelectric element system in resonance mode. With selective recording of electrode vibrations at a given frequency, the electrode elongation of the order of thousandths of an atom’s size, that is 10 ^-11 cm, becomes noticeable.The ratio of small amplitudes of surface tension and surface charge density is recorded as a derivative of the surface tension of a solid with respect to surface charge density.

The potential of the test electrode, measured relative to the standard reference electrode, varies linearly with time. An oscillogram of the dependence of the indicated derivative on the potential is recorded. The scope of the waveform is determined using a reference thermal tension created by alternating heating of the surface of the electrode under study. The recorded dependence of the estance potential contains information about surface phenomena and is not subject to the influence of volumetric processes, as well as to the influence of the macroscopic shape of the electrode.

A method of demonstrating the self-isolation of a metal and the splitting of its surface charge is not known.

The present invention has the task of experimentally showing the relationship between the spontaneous formation of an insulating coating on an oxide-free metal and the splitting of the surface charge of the metal into two parts separated by this insulating coating.

The problem is solved due to the fact that according to the invention, the electrode from the metal under study is brought into contact with the ionic liquid, the dependences of the estance and electric current density on the electrode potential are recorded on the indicated electrode, steps in the form of extremum and kinks are found on the dependence of the estance on the potential, located in the range of potentials, where the dependence of the density of the electric current on the potential, changing in the anode direction, has no extremum or kinks, evaluate the dimensionless the slope of the estance-potential dependence between the stages of estance shows the superiority of the maximum absolute value of this slope over unity, from this superiority we conclude that the metal has an insulating coating, which is composed exclusively of metal atoms and whose electrostriction increases the absolute value of the indicated slope, and the absence of density changes the electric current corresponding to the stages of estance is explained by the splitting of the surface charge of the metal with an insulating coating on the inside the back and outer parts, which, when the potential changes, in turn receive charge increments with the formation of mutually compensating steps, and of these parts, only the internal, step-changing part causes electrostriction in the insulating coating, which creates estance steps, while the electric current is caused by a change in the total surface charge not having steps.

The dependence of estans on potential is recorded in the coordinates "module of estans - potential." The range of potential change includes the polarizability region of the electrode.

As the studied metal, platinum is used. In this case, an aqueous solution of sulfuric acid with a concentration of 0.5 mol per liter is used as the ionic liquid, the dependences of estans and electric current density on the potential are recorded in the potential range, including the interval from +0.8 V to +0.3 V relative to the equilibrium hydrogen electrode when the potential in the anode changes direction. Potassium fluoride or sodium fluoride is also used as an ionic liquid.

As the studied metal, copper is used. In this case, an aqueous solution of sodium fluoride with a concentration of 0.1 mol per liter is used as the ionic liquid, the dependences of estans and electric current density on the potential are recorded in the potential range, including the interval from -1.8 V to -0.4 V relative to the saturated calomel electrode when the potential in the anode changes direction.

A comparison of the dependences of estans and electric current on potential indicates a paradoxical difference between two physical quantities characterizing the state of the same surface. According to the oscillogram of estance, a change in potential is accompanied by a series of sharp changes in the state of the surface, which are expressed by large steps of estance. According to the oscillogram of the electric current, there is no series of changes in the state of the surface at the same potentials, which is expressed by the fairly smooth course of the dependence of the electric current on the potential.

The indicated paradoxicality is sufficiently clear and is used in the present invention to demonstrate the splitting of the surface charge of a metal with an insulating coating formed by surface metal atoms with an electron deficiency.

The total excess surface charge of the metal occurs as a reaction to an external electric field created in this case by ions of an electrolyte solution. In this case, the total surface charge is split into an internal partial charge located below the insulating coating, and an external partial charge located above the insulating coating. Part of the external electric field ends at the external partial charge and does not reach the insulating coating. The rest of the external electric field passes through the insulating coating and ends on the internal partial charge. Thus, only the internal partial charge is involved in creating an electric field that causes electrostriction of the insulating coating and contributes to the change in the surface tension of the solid.

When the potential changes, the total surface charge monotonically changes, while the internal and external partial charges increment in turn, which creates mutually inverse steps for them, which do not affect the total surface charge. The paradox is explained by the difference in the parts of the charge that affect the change in estance and electric current with potential. The stages of estance reflect the stepwise dependence of the internal partial charge on the potential. Electric current is determined by a stepless change in the total surface charge.

Along with the estance steps, an abnormally high dimensionless slope of the estance-potential dependence in the areas between the estans steps is an independent confirmation of the existence of its own insulating coating, which consists exclusively of metal atoms. He shows that in the surface tension there is a significant contribution of electrostriction, which is characteristic of dielectrics in an electric field. The creation of an insulating coating only by metal atoms, without the formation of oxide, allows us to consider this phenomenon as self-isolation of metal.

обусловлено отсутствием у этих анионов существенной способности к хемосорбции, что обеспечивает электростатический характер заряжения ионной обкладки двойного электрического слоя на границе твердой и жидкой фаз.The use of ionic liquids with anions F ^- and

due to the absence of a significant chemisorption ability in these anions, which ensures the electrostatic nature of the charging of the ionic lining of the double electric layer at the interface between the solid and liquid phases.

The illustrations show:

FIG. 1 - oscillogram of the estance potential obtained on platinum in the cathode and then anode directions.

FIG. 2 - waveform current density-potential obtained on platinum in the cathode and then anode directions.

FIG. 3 - combined oscillograms of the estance potential and current potential obtained on copper in the anode and then cathode directions

FIG. 4 - discrete shifts of the dependence of the estance potential on copper, due to the stepwise change in the internal partial charge of the metal surface. The dimensionless slope of the estance-potential dependence is approximately the same before and after each shift, expressed by the step of estance in the potential region located more than zero of the estance. When passing through zero the estance in the positive direction, the slope of the dependence of the estance potential decreases one and a half times, but remains significant.

обусловлено снижением R_m. Приведенные графики рассчитаны для указанных параметров эквивалентной схемы, различающихся значениями R_m.FIG. 5 is an equivalent circuit of a double electric layer at the interface between a solid metal and a surface-inactive ionic liquid. The capacitance C _{m of the} insulating coating is closed by a constant resistance R _m and an interrupted resistance R _o simulating a quantized bypass. Observed in some cases, the blurring of zero estans on the oscillograms

due to a decrease in R _m . The graphs are calculated for the indicated parameters of the equivalent circuit, differing in the values of R _m .

FIG. 6 is a schematic sectional view of a surface layer of a metal with a positive total excess surface charge. The increase in the total density of the surface charge q = q _in + q _ex (shift to more positive values) occurs due to the exit of electrons from the surface layer of the metal when the middle position of the positive metal ions is fixed at the nodes of the crystal lattice. An increase in the external and internal partial charges q _ex and q _in also occurs by the loss of electrons by them. Losses are shown, respectively, by long and short solid arrows. The dotted arrow indicates reverse shunting. Circles with a minus sign indicate excess electrons. Circles with a plus sign indicate excess metal ions in its crystal lattice.

FIG. 7 is a schematic sectional view of a surface metal layer at zero total excess surface charge. A zero external partial charge is shown without an electron cloud, which corresponds to a charge-compensated intrinsic dipole jump of the metal.

FIG. 8 is a schematic sectional view of a metal surface layer with a negative total excess surface charge, both parts of which (internal and external) are formed by electrons.

FIG. 9 is a diagram of the formation of steps of internal and external partial charges separated by an insulating coating. Mutual compensation of the steps excludes their influence on the total surface charge.

FIG. 10 is a diagram of the formation of a stepwise dependence of the estance potential (UPABCDEFGHO) by repeating the steps of the internal partial charge.

The following notation is used in the illustrations and description:

γ is the surface tension of a solid in the form of an electrode,

q is the total surface density of the excess charge of the electrode,

q = q _in + q _ex ,

own dipole jump of the metal does not contribute to the excess charge of the electrode,

q _in - internal partial charge (part of the charge density q located under the insulating coating),

q _ex - external partial charge (part of the charge density q located above the insulating coating),

φ is the electrode potential measured relative to the reference electrode,

ϑ is the relative change in the area of the interface between the electrode and the ionic liquid during elastic deformation of an electrode having an isotropic polycrystalline surface,

∂γ / ∂q - estans,

- модуль (абсолютная величина) эстанса,

- module (absolute value) of estance,

φ _Z is the zero of estance, that is, the potential corresponding to the passage of estans through zero,

γ _qφ = ∂ (∂γ / ∂q) / ∂φ is the dimensionless slope of the estance potential dependence,

- модуль безразмерного наклона,

- dimensionless tilt module,

j is the density of the electric current through the boundary of the electrode with the ionic liquid,

C = ∂q / ∂φ is the specific differential capacitance of the boundary of the electrode with the ionic liquid (hereinafter, specific values are assigned to a unit of the area of the phase boundary),

C _s - specific differential capacitance of the ionic part of the double layer,

C _m - specific differential capacity of the metal part of the double layer,

R _o - specific interruptible resistance of the insulating coating,

R _m is the specific constant resistance of the insulating coating,

φ _a is the potential jump in the double electric layer at the phase boundary,

RVE - equilibrium hydrogen electrode,

NKE - saturated calomel electrode,

in kHz, the frequency of oscillations of the surface tension and surface charge density is indicated when registering the estans as the ratio of the amplitudes (Δγ and Δq) of these quantities, ∂γ / ∂q → Δγ / Δq as Δγ, Δq → 0.

Below are two examples of using the proposed method.

от потенциала φ, измеренного относительно равновесного водородного электрода (РВЭ) в том же растворе. Потенциал φ изменяют со скоростью 0.1 В/с сначала в катодном направлении, от +1.64 В до 0 В, затем в анодном направлении, от 0 В до +1.64 В (Фиг. 1). В тех же границах и в той же последовательности регистрируют зависимость плотности электрического тока j от потенциала φ при скорости изменения потенциала 0.2 В/с (Фиг. 2). Область поляризуемости платинового электрода ограничена волной разряда ионов водорода и волной анодного окисления металла. Данная область входит в указанный диапазон изменения потенциала.Example 1. The platinum electrode is brought into contact with an aqueous solution of sulfuric acid with a concentration of 0.5 mol per liter at a temperature of 20 ± 2 ° C. The dependence of the modulus of estance is recorded on this electrode

from the potential φ, measured relative to the equilibrium hydrogen electrode (RWE) in the same solution. The potential φ is changed at a speed of 0.1 V / s, first in the cathode direction, from +1.64 V to 0 V, then in the anode direction, from 0 V to +1.64 V (Fig. 1). At the same boundaries and in the same sequence, the dependence of the electric current density j on the potential φ is recorded at a potential change rate of 0.2 V / s (Fig. 2). The polarizability region of the platinum electrode is limited by the wave of the discharge of hydrogen ions and the wave of anodic oxidation of the metal. This area is included in the indicated range of potential changes.

Three stages are found on the dependence of estans on potential: one step M1 in the form of an extremum (maximum modulus of estans) and two steps M2, M3 in the form of kinks (Fig. 1). The found estance steps are located in the potential region from φ _K = + 0.35 V to φ _A = + 0.83 V, in which the dependence of the electric current density on the potential changing in the anode (positive) direction has no extremum or kinks (Fig. 2 ) The indicated range of potentials is limited by the processes of platinum oxidation (for φ> φ _A ) and hydrogen adsorption (for φ <φ _K ).

The potential of the zero charge of platinum is located more negative than this region, which therefore corresponds to a positive surface charge of the metal.

. Отсюда следует неравенство

, показывающее значительное превосходство максимальной абсолютной величины безразмерного наклона над значением 1. При интерпретации полученного неравенства исходят из уравнения безразмерного наклонаDimensionless measured inclination _{γ qφ = ∂ (∂γ / ∂q} ) / ∂φ depending estance potential between said steps estance and estimate its absolute maximum value. Between steps M2 and M3, the dimensionless slope is negative and reaches the value ∂ (∂γ / ∂q) /∂φ=-6.6. Accordingly, the absolute value of the indicated dimensionless slope reaches a value

. This implies the inequality

, showing a significant superiority of the maximum absolute value of the dimensionless slope over the value of 1. When interpreting the resulting inequality, we proceed from the equation of the dimensionless slope

∂ (∂γ / ∂q) / ∂φ = -1- (∂С / ∂ϑ) / С- (∂γ / ∂q) (∂C / ∂φ) / С,

deduced in the cited book of the author. The term -1 reflects the contribution of electrostatic repulsion along the double electric layer to the surface tension. The remaining members describe more complex interactions.

The absence of kinks on the oscillogram of the current density in the region of the estance steps indicates a small value (∂С / ∂φ) / С, which characterizes the change in the capacitive current with potential. In addition, in the vicinity of the zero estans, where ∂γ / ∂q = 0, the quantity ∂γ / ∂q is small. This allows us to neglect the last term of the indicated equation in the region of estance steps and use the simplified equation

∂ (∂γ / ∂q) / dφ = -1- (∂С / ∂ϑ) / С.

Substitute the found value ∂ (∂γ / ∂q) /∂φ=-6.6 into this equation and find the relative derivative of the differential capacity with respect to elastic deformation (∂С / ∂ϑ) /С=5.6. The difference of this quantity from zero and, moreover, the significance of this quantity indicate the relation between the estance steps and electronic processes in the surface layer of the metal, since at the interface of a solid metal with an ionic liquid, only metal is subject to elastic deformation. The possibility of a substantial participation of adsorbed ions and liquid molecules in elastic deformation is excluded by the absence of changes in the electric current that would correspond to the observed steps of estance.

From the found value (∂С / ∂ϑ) / С it follows that a certain electric capacitance C _{m is} present in the surface layer of the metal, supplementing the electric capacitance C _{s of the} ionic part of the double electric layer to the total capacitance C,

C = C _s C _m / (C _s + C _m ).

Based on the existence of an electric capacitance C _m having an insulating gap, it is concluded that the metal has an insulating coating, the electrostriction of which increases the absolute value of the dimensionless slope of the estance-potential dependence between the steps of the estance.

The origin of the estance steps is explained by the splitting of the surface charge into two parts located on both sides of the insulating coating, under it and above it, respectively, the internal and external partial charges.

Given the absence of changes in the density of the electric current corresponding to the steps of estance, they conclude that the indicated parts of the surface charge alternately increase when the potential changes, which creates mutually compensating steps for them that do not affect the total surface charge, the change of which creates an electric current.

Of the two parts of the surface charge, only the internal partial charge induces the electrostriction of the insulating coating, which contributes to the surface tension and to the recorded estance. Under these conditions, the dependence of estans on potential repeats the steps of the internal partial charge.

, и плотность тока-потенциал, j - φ, которые совмещают на одной фотографии при сохранении общего начала отсчета потенциала (Фиг. 3). Потенциал изменяют со скоростью, соответственно, 0.1 В/с и 0.2 В/с, сначала в катодном и затем в анодном направлениях. Зависимость эстанса от потенциала снимают в интервале между -0.33 В и -1.82 В, который включает потенциал нуля эстанса φ_Z, практически совпадающий с потенциалом нулевого заряда меди. Область положительного поверхностного заряда металла расположена в данном случае при φ>φ_Z.Example 2. The copper electrode is brought into contact with an aqueous solution of sodium fluoride with a concentration of 0.1 mol per liter at a temperature of 20 ± 2 ° C. The electrode potential is measured relative to the saturated calomel electrode (NCE). The oscillograms of the estance-potential module are sequentially recorded.

, and the current density potential, j - φ, which are combined in one photograph while maintaining the common reference point of the potential (Fig. 3). The potential changes with a speed of 0.1 V / s and 0.2 V / s, respectively, first in the cathode and then in the anode directions. The dependence of the estans on the potential is removed in the interval between -0.33 V and -1.82 V, which includes the zero potential of the estance φ _Z , which practically coincides with the zero charge potential of copper. The region of the positive surface charge of the metal is located in this case for φ> φ _Z.

On the dependence of estans on the potential obtained in the anode direction, four steps are found: one step M1 in the form of an extremum (maximum modulus of estans) and three steps M2, M3, M4 in the form of kinks (Fig. 3). The found estance steps are located in the potential region from φ _Z = -1.30 V to φ _M = -0.48 V, in which the dependence of the electric current density on the potential changing in the anode (positive) direction has no extremum or kinks.

Within the interval φ _M >φ> φ _{Z, the} stages of estance are almost completely reversible. A certain discrepancy between the forward and reverse motion of the estance waveform, which is observed in the above illustration (Fig. 3), is caused by the potential sweep beyond the specified limits and the beginning of the anodic oxidation of copper at a potential of -0.33 V. On the oscillogram of the current density, the oxidation of copper is expressed by the anode wave reaching at -0.33 V maximum (facing down).

, показывающее значительное превосходство максимальной абсолютной величины безразмерного наклона над значением 1.The dimensionless slope γ _qφ = ∂ (∂γ / ∂q) / ∂φ of the dependence of the estance potential between the indicated steps of the estance is measured and its maximum absolute value is estimated. Between steps M2 and M3, the dimensionless slope is negative and reaches the value ∂ (∂γ / ∂q) /∂φ=-3.3. This implies the inequality

, showing a significant superiority of the maximum absolute value of the dimensionless slope over a value of 1.

The interpretation of the inequality obtained is carried out similarly to the case of platinum considered. It is concluded that there is an insulating coating formed by copper atoms and splitting the surface charge of the metal into two parts, which change stepwise with the potential. It is shown that only the inner part of the surface charge causes electrostriction of the insulating coating. The appearance of estance steps is explained by the stepwise dependence of the internal part of the surface charge on the potential.

The incorporation of an insulating coating into a surface metal layer is shown schematically in FIG. 5-7 at zero, negative and positive total surface charge. The correspondence between the estance steps and the inner part of the surface charge is shown in FIG. 8, 9.

The physical cause of the quantized sequential increment of two parts of the surface charge separated by an insulating coating remains a mystery. However, the fact of the existence of such a quantized process is experimentally confirmed by a series of stages of estance similar to each other on different metals.

Claims (11)

1. A method of demonstrating the self-isolation of a metal and the splitting of its surface charge, characterized in that the electrode from the metal under study is brought into contact with an ionic liquid, the dependences of estance and electric current density on the electrode potential are recorded on the indicated electrode, steps in the form are found on the dependence of estance on the potential extremum and kinks located in the range of potentials where the dependence of the electric current density on the potential, changing in the anode direction, has no extremum, no kinks, evaluate the dimensionless slope of the dependence of the estance-potential between the steps of the estance, show the superiority of the maximum absolute value of this slope over unity, from the indicated superiority we conclude that the metal has an insulating coating, which is composed exclusively of metal atoms and whose electrostriction increases the absolute value of the specified slope , and the absence of changes in the electric current density corresponding to the steps of estance is explained by the splitting of the surface charge metal with an insulating coating on the inner and outer parts, which, when the potential changes, alternately receive charge increments with the formation of mutually compensating steps, and of these parts, only the internal, stepwise changing part causes electrostriction in the insulating coating, creating estance steps, while the electric current is caused by a change in the total surface charge without steps. 2. The method according to p. 1, characterized in that the dependence of the estans on the potential is recorded in the coordinates "module of estans - potential". 3. The method according to p. 1, characterized in that in the range of potential changes include the polarizability region of the electrode. 4. The method according to p. 1, characterized in that platinum is used as the test metal. 5. The method according to p. 4, characterized in that an ionic liquid is used an aqueous solution of sulfuric acid with a concentration of 0.5 mol per liter, the dependence of estans and electric current density on the potential is recorded in the potential range, including the interval from +0.8 V to +0.3 In a relatively equilibrium hydrogen electrode when the potential changes in the anode direction. 6. The method according to p. 4, characterized in that the ionic liquid is a melt of potassium fluoride or sodium fluoride. 7. The method according to p. 1, characterized in that as the investigated metal using copper. 8. The method according to p. 7, characterized in that an ionic liquid is used an aqueous solution of sodium fluoride with a concentration of 0.1 mol per liter, the dependence of estans and electric current density on the potential is recorded in the potential range, including the interval from -1.8 V to -0.4 In a relatively saturated calomel electrode when the potential changes in the anode direction. 9. The method according to p. 1, characterized in that silver is used as the test metal. 10. The method according to p. 1, characterized in that the quality of the investigated metal using gold. 11. The method according to p. 1, characterized in that nickel is used as the test metal.

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