ELECTROCHEMICAL SYNTHESIS OF IRON MONOSELENIDE THIN FILMS Текст научной статьи по специальности «Химические науки»

262 CHEMICAL PROBLEMS 2021 no. 4 (19) ISSN 2221-8688

<S

UDC 541.13.544.65

ELECTROCHEMICAL SYNTHESIS OF IRON MONOSELENIDE THIN FILMS

A.O. Zeynalova, S.P. Javadova, V.A. Majidzade, A.Sh. Aliyev

Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry of the National Academy of Sciences AZ1143, H.Javid ave. 113, Baku, Azerbaijan e-mail: vuska_80@mail.ru

Received 18.10.2021 Accepted 09.12.2021

Abstract: In the presented research work, the kinetics and mechanism of the deposition process of thin iron, selenium and Fe-Se films have been studied by recording a cyclic and linear polarization curves by potentiodynamic method using Pt and Ni electrodes. Individual and co-deposition potential areas of the components of the electrolyte on the Pt electrode were determined. In order to determine the optimal electrolysis mode and electrolyte composition, the effect of various factors (concentration of initial components, temperature, etc.) on the co-electrodeposition process of Fe-Se was studied. In addition, Fe-Se samples deposited on the surface of Ni electrodes were thermally treated at 4500C and studied by SEM and X-ray phase analysis methods. Elemental analysis of the films shows that they contain 42.2% Fe and 57.8% Se.

Keywords: electrodeposition, iron monoselenide, polarization, electro-reduction DOI: 10.32737/2221-8688-2021-4-262-271

Introduction

In our time, the great demand for renewable energy sources and energy storage devices with high-efficiency grows day by day [1-2]. Thin films of metal chalcogenides are widely used in these devices [3-6]. In this regard, in the last few years, iron chalcogenides attract attention due to their structure, the band gap, physicochemical, magnetic and etc. properties.

Iron monoselenide can be obtained variously. In some studies, FeSe thin films were deposited on a conductive glass electrode coated with fluorine-additive tin oxide by electron-beam evaporation of Fe(NO3)-9H2O in the range of 0.18-0.03M concentration. The effect of concentration on the thickness, structure, surface morphology, composition and optical properties of deposited samples were studied [712]. Nanocrystalline iron monoselenide was synthesized on the surface of platinum or gold electrodes from aqueous acid solutions containing HSeO3-, SeO32- and Fe2+ ions by potentiostatic or galvanostatic methods. The current density and potential values providing the deposition of tetragonal iron selenide were

determined by electrochemical method [13, 14].

FeSe samples consisting of polygonal crystallites were synthesized by [13-15] authors. Atomic force microscopy were used to determine the optical properties and UV-VIS spectroscopy was applied to study the surface topography of thin films. According to the results, the bandgap of the deposited films is 1.23 eV, and as the deposition time increases, the bandgap of the deposited thin films decreases. Detailed study of the samples by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersion spectroscopy shows that they are rich in amorphous selenium. High abundance of selenium makes it possible to synthesize good crystalline, tetragonal FeSe2.

Selenides of transition metals are widely studied as anode materials for batteries and as an effective electro-catalyst in photo-electrochemical decomposition of water. Iron chalcogenides are widely investigated for different applications, such as hydrogen evolution reaction, light energy conversion devices, solar cells, superconductors, high-

CHEMICAL PROBLEMS 2021 no. 4 (19)

www.chemprob.org

capacity capacitors (supercapacitors) and storage devices. In terms of usefulness, these are mostly studied as photovoltaic or supercapacitors [15, 16].

Analysis of reference data shows that Fe-Se thin films were mainly synthesized from

acid solutions [13, 15, 17, 18]. Therefore, the main purpose of this study - as a continuation of our research about the synthesis of metal chalcogenides [19-22] - is to obtain thin iron selenium films from aqueous solutions using a simple and inexpensive electrochemical method.

Experimental part

Pt and Ni electrodes were used in galvanostatic synthesis and potentiodynamic research, and they were connected to an ampermeter for regulating the current. The thin films obtained by this method were thermally treated at 4500C in an argon medium. It was observed that the obtained thin film changed from an amorphous structure to a crystalline structure.

Potentiodynamic studies were performed using IviumSoft-programmed and computer-equipped "IVIUMSTAT

Electrochemical Interface" potentiostat. In this case, a three-electrode electrochemical cell with a volume of 100 ml was used. Pt wire with an area of 0.3 cm2 and Ni plate with an area of 2 cm2 were used as working electrodes. Saturated silver/silver chloride (Ag/AgCl/KCl) electrode

was used as the reference electrode, and Pt plate with an area of 4 cm2 applied as the auxiliary electrode.

The morphological structure and phase composition of the samples were studied by the scanning electron microscope (SEM) and "D2 Phazer" (filter CuKa, Ni) X-ray phase analyzer of German company Bruker, respectively.

Electrolyte solutions containing 0.1 M Fe(N03)3 ■ 9H20 and 0.01 M H2Se03 were used during the experiments. After cleaning the Pt electrode in concentrated nitric acid, it is washed with distilled water. The Ni electrode is polished chemically in nitric acid and then electrochemically in a solution consisting of concentrated sulfuric acid (H2SO4), orthophosphate acid (H3PO4) and distilled water.

Results and discussion

The deposition potential area for both iron (Fe) and selenium (Se) was determined by studying the electrochemical reduction process of the initial components separately to carry out the co-deposition process by electrochemical method. Initially, the electro-reduction potential area of iron ions in an aqueous electrolyte was specified. The experiments were performed by

the potentiodynamic method recording a polarization curve on the surface of the Pt electrode.

Fig. 1 shows the mechanism of the electrochemical reduction process of Fe ions on the Pt electrode. As is seen from the Fig., the process occurs through the following scheme in two stages within 0.6 - (-0.9) V potential range:

Fe (III) ^ Fe(II) ^ Fe (0)

Here, electroreduction to divalent iron ions occurs in the potential range of 0.6 - 0.1V (1), and electro-reduction to neutral iron occurs

in the range of -0.2 reaction [23]:

(-0.5) V according to (2)

Fe3+ + e" = Fe2+ Fe2+ + e" = Fe

(1) (2)

Here, after -0.5 V potential, the surface of the Pt passive, and thereby the current value decreases. electrode is covered with iron and it becomes

Fig. 1. Cyclic polarization curve of the electro-reduction process of iron ions in an aqueous electrolyte on the Pt electrode. Electrolyte (M): 0.1 Fe(N03)3 ■ 9H20 + H2O, T = 298K, Ev = 0.02 V/sec.

Then, starting from the -0.9 V potential, the free iron ions are evenly distributed on the surface of the electrode, and thus the current consumed in the process increases sharply due to the thickening of the layer deposited on the

electrode surface. After the potential of -1.0 V, the release of hydrogen gas is observed in the electro-reduction process which affects the process, so the process was studied to the potential of -1.0 V.

' ' ' is ' ' ' uT Potential

Fig. 2. Cyclic polarization curve of the electro-reduction process of selenium ions from an aqueous electrolyte on a Pt electrode. Electrolyte (M): 0.1 M H2SeO3, T = 298K, Ev = 0.02V/sec

The mechanism of the electrochemical reduction process of selenite ions proceeds in reduction process of selenite ions on the Pt two stages in the potential range of 0.5 - 0.1 V electrode is shown in Fig.2. The electro- in line with the following scheme:

Se (IV) ^ Se (0) ^ Se (II)

Initially, the reaction (3) occurs starting from complete reduction of the selenide ions occurs the stationary potential (0.5 V) to the 0.1 V according to the reaction (4) potential, and after the 0.1 V potential, the

SeO32- + 7H+ +6e = HSe + 3H2O (3)

SeO32- + 6H+ +6e = Se2- + 3H2O (4)

After determining the deposition potential area of initial components and the mechanism of the electro-reduction process, their electrochemical co-deposition was

performed. In order to carry out this process, numerous studies were carried out and a combination of stoichiometric composition and its thin films obtained. The research work was

performed on both Pt (Fig. 3) and Ni (Fig. 4) electrodes.

o

0.0

Potential

Fig. 3. Cyclic polarization curve of the electrochemical co-deposition process of Fe and Se from an aqueous electrolyte on the Pt electrode. Electrolyte (M): 0.035 Fe (NOb)B + 0.0025 H2SeO3; T = 298 K, EV = 0.02 V/sec.

As is seen from Fig. 3, the co-deposition process occurs at a potential of 0.65 - (-0.6) V. Here, the potential range of 0.65 - (-0.38) V indicates the electro-reduction of selenite ions. After -0.38 V potential, the Fe-Se films were deposited due to the combination of iron ions

with selenide ions formed in the electrolyte. After a potential of -0.38 V, the surface of the Pt electrode was covered with a gradually compacting layer of Fe-Se, which led to a thickening of the film. Thus, a sharp increase in current was observed (Fig. 3).

Fig. 4. Cyclic polarization curve of the electrochemical co-deposition process of Fe and Se from an aqueous electrolyte on the Ni electrode. Electrolyte (M): 0.035 Fe (NO3)3 + 0.0025 ^SeO3; T = 298 K, Ev = 0.02 V/sec.

The electrochemical co-deposition process of iron and selenium on the surface of the electrode was slightly different (Fig. 4). Thus, starting from the 0.0V potential, the -0.3 V potential range accords with the reduction of selenide ions to selenium. Starting from the -0.3V potential, selenium atoms were reduced to selenide ions and the obtained selenide ions combine with iron ions on the surface of the electrode to form a thin iron monoselenide film.

The electrochemical deposition of iron

monoselenide was also confirmed by X-ray and SEM analyzes of samples obtained on the Ni electrode.

Fig.5 shows the X-ray phase analysis of Fe-Se sample obtained from an aqueous electrolyte on the surface of Ni electrode. It is seen from the Fig. that the intensities of the diffraction peaks of this sample were very weak. The reason of this is the amorphous structure of the deposited iron monoselenide.

Fig. 5. X-ray phase analysis of Fe-Se sample from aqueous electrolyte on Ni electrode. Electrolyte

(M): 0.035 Fe(NO3)3 + 0.0025 №SeO3; Т=298 K

e (NR), ( Fe , Ni ), 00-037-0474

Nickel, syn, Ni, 01-070-0989

100 50

J_L

n Selenide, Fe Se, 01-073-6936

30

2-theta (deg)

6.0e+004

4.0e+004

2.0e+004

0.0e+000

30

50

60

100

0 100

0

10

Fig. 6. X-ray phase analysis, morphology and element composition of the FeSe sample deposited on the Ni electrode. Electrolyte (M): 0.035 Fe(NO3> + 0.0025 №SeO3; t=30 min.

1.0e+004 8.0e+003 6.0e+003 4.0e+003 2.0e+003 0.0e+000

Nickel, syn, Ni, 01-071-4653

cubic selenium, syn, Se, 01-071-:

Penroseite, syn, Ni Se2, 01-071-4944

._I Ni I I 4

i Selenide, Fe Se, 03-065-9120

Achavalite, syn, Fe Se, 01-075-

Fig. 7. X-ray phase analysis, morphology and element composition of the FeSe sample deposited on the Ni electrode; Electrolyte (M): 0.035 Fe(NOs)3 + 0.0025 H2SeOs; t=60 min.

In order to increase the crystallinity of the samples, they were thermally processed in an Ar atmosphere at 4500C temperature for 0.5-1 hour (Fig.s 6, 7). The results of repeated X-ray phase analysis show that more satisfying crystals are observed in the heat-treated sample for 1 hour.

The thickness of the deposited thin films is 4-6 microns. According to the results of EDS (element composition) analysis, the composition of the films corresponds to the stoichiometry and they consist of 42.2% Fe and 57.8% Se.

0

10

20

30

50

2-theta (deg)

Potential v

Fig. 8. The effect of temperature on the process of electrochemical co-deposition of Fe and Se from an aqueous electrolyte on the Pt electrode; Electrolyte (M): 0.035 Fe(NOs)3 + 0.0025 H2SeOs; Ev=0.02 V/s. T(K): 1- 298; 2 - 308; 3 -318; 4 - 328; 5 - 338; 6 - 348

The effect of some important factors on composition and electrolysis condition for the the electrochemical deposition process was deposition of iron monoselenide thin films. The studied to determine the optimal electrolyte effect of temperature on the co-deposition

process was studied in the range of 298-348 K (Fig. 8). As is seen from the figure, the increase in temperature has a positive effect on the electrodeposition process. Thus, at a temperature of 298 K and at 348 K, the stationary potential is 0.67V and 0.72V, respectively. That is, with the positive effect of temperature, the co-deposition potential shifts to the direction of the positive potential area of

Potential v

0.05V. However, the increase in temperature has a negative effect on the quality of the deposited films. Thus, their adhesion deteriorates, they begin to break off the electrode surface, and the composition of the films deviates from the stoichiometry. Therefore, 298-308 K was chosen as the optimal temperature range.

Potential

Fig. 9. The effect of concentration of ions on the electrochemical co-deposition process of Fe and Se from an aqueous electrolyte on the Pt electrode; Electrolyte (M): 1- 0.025; 2- 0.035; 3- 0.045; 40.055 Fe(NO3)3 + 0.0025 M №SeO3 (Fig. 9 a); 0.035 Fe(NO3)3+ 1- 0.0005; 2- 0.0025; 3- 0.005; 40.0075 H2SeO3(Fig. 9 b); Ev=0.02 V/s. T = 298 K

The effect of Fe and Se on the codeposition process of the initial ion concentration (Fig. 9 a, b) and the rate of potential change (potential gradient) (Fig. 10) were also studied by recording linear polarization curves. When studying the effect of concentration, first, the concentration of selenite ions in the solution is kept constant. The concentration of iron ions was studied in the

range of 0.025-0.055 M (Fig. 9 a). The concentration of selenite ions was examined in the range of 0.0005-0.0075M at the constant concentration of iron ions (Fig. 9 b). As is seen from the figure, in both cases there is no significant sharp potential in the electrochemical deposition process, only the current required to the process (0.5 mA) slightly increases.

0 0_

-0,5. //

-1.0. -1.5.

-2.0. 1//'

-2 5. 6 V7 ~~

1 1 1 > 1 -0.5 0 ■ 1 1 1 ■ 0 0.5

Fig. 10. Cathode polarization curves of the electrochemical deposition process of Fe and Se from an aqueous electrolyte on the Pt electrode at different rates of potential change (1- 0.005; 2- 0.02; 3- 0.04; 4- 0.06; 5- 0.08; 6- 0.1). Electrolyte (M): 0.035 M Fe(NO3)3 x 9H2O + 0.0025 M №SeO3

Potential

The effect of rate of potential change on the codeposition process was studied in the range of 0.005 - 0.08V/s (Fig. 10). As is seen from the figure, despite the fact that the study of the rate

of potential change does not show a positive potential shift, there is an increase in the current consumed in the electrodeposition process. This increase is approximately 0.8 mA.

Conclusion

In this work, the separate electroreduction process of iron and selenium on the surface of the Pt electrode was studied by the electrochemical method and their deposition potential areas determined. The kinetics and mechanism of the deposition process of Fe-Se thin films were studied by potentiodynamic methods and recording cyclic and linear polarization curves using Ni electrodes. The effect of various factors (concentration of initial

components, temperature, etc.) on the co-electro-deposition process was studied to determine the optimal electrolysis mode and electrolyte composition. Besides, Fe-Se samples deposited on the surface of Ni electrodes were thermally treated at 45 00C and studied by SEM and X-ray phase analysis methods. Elemental analysis of the films shows that they contain 42.2% Fe and 57.8% Se.

References

1. Fateev V.N., Alexeeva O.K., Korobtsev S.V., Seregina E.A., Fateeva T.V., Grigorev A.S., Aliyev A.Sh. Problems of accumulation and storage of hydrogen. Chemical Problems, 2018, no. 4, pp. 453-483.

2. Kulova T.L., Nikolaev I.I., Fateev V.N., Aliyev A.Sh. Modern electrochemical systems of energy accumulation. Chemical Problems, 2018, no. 1, pp. 9-34.

3. Aliyev A.Sh., Elrouby M., Cafarova S.F., Electrochemical synthesis of molybdenum sulfide semiconductor. Mat. Sci. Semicon. Proc. 2015, vol. 32, pp. 31-39.

4. Javadova S.P., Majidzade V.A., Aliyev A.Sh., Tagiyev D.B. Effect of major factors on the composition of thin Bi2Se3 films. Russian journal of applied chemistry, 2021, vol., 94, no.1, pp. 38-42.

5. Jafarova S.F., Majidzade V.A., Kasimogli I., Eminov Sh.O., Aliyev A.Sh., Azizova A.N., Tagiyev D.B. Electrical and Photoelectrochemical Properties of Thin MoS2 Films Produced by Electrodeposition. Inorganic Materials, 2021, vol., 57, no. 4, pp. 331-336.

6. Aliyev O.M., Maksudova T.F., Azhdarova D.S., Mamedov Sh.G., Gamidova Sh.A.

The Bi2S3 - YbS system. Chemical Problems, 2021, no. 3, pp. 168-172.

7. Segu Sahuban Bathusha N.M., Chandra Mohan R., Saravana Kumar S.,

Ayeshamariam A., Jayachandran M. Micro Structural and Optical Properties of Ferrous Selenide Thin Films and Its Characterization. FluidMech Open Acc. 2017, vol. 4, issue 2, 6 pages.

doi: 10.4172/2476-2296.1000156

8. Sohrabi P., Ghobadi N. Optical and photocatalytic behaviors of iron selenide thin films grown by chemical bath deposition versus deposition time and annealing temperature. Applied Physics A, 2019, vol. 125, no. 9, pp.1-11. DOi: 10.1007/s00339-019-2919-8

9. Obata Y., Sato M., Kondo Y., Yamaguchi Y., Karateev I.A., Pavlov I., Vasiliev A.L., Haindl S. Chemical Composition Control at the Substrate Interface as the Key for FeSe Thin-Film Growth. ACS Appl. Mater. Interfaces . 2021, vol. 13, 44, pp. 5316253170.

10. Laurinavichyute, V.K., Bakhtenkova, S.E., Drozhzhin, O.A., Kazakov, S.M., Antipov, E.V. Electrodeposition of FexSey films from acidic solutions. Russian Journal of Electrochemistry, 2016, vol. 52(11), pp. 10481056. doi:10.1134/s1023193516110082

11. Demura S., Tanaka M., Yamashita A., Saleem J.D., Hiroyuki O., Masaya F., Yamaguchi T., Takeya H., Iida K., Holzapfel B., Sakata H., Takano Y.

Electrochemical Deposition of FeSe on RABiTS Tapes. Journal of the Physical Society of Japan, 2016, vol. 85, No. 1. https://doi.org/10.7566/JPSJ.85.015001

12. Chen, P.Y., Hu, S.F., Liu, R.S., & Huang, 18. C.Y. Electrodeposition of nano-dimensioned FeSe. Thin Solid Films, 2011, vol. 519, no.

23, pp. 8397-

8400. doi:10.1016/j.tsf.2011.04.002

13. Kumar R., Mitra A., Varma G.D. Vortex 19. state study of superconducting Fe(Te, Se) thin films deposited on SrTiO3 substrate by pulsed laser deposition technique. AIP Advances, 2019, vol. 9, p. 125128, doi.org/10.1063/1.5129605

14. Pesko E., Zukowska G., Zero E., Krzton-Maziopa A. Electrocrystallization of 20. nanostructured iron-selenide films for potential application in dye sensitized solar cells. Thin Solid Films, 2020, vol. 709, p.138121.

doi.org/10.1016/j.tsf.2020.138121

15. Oyetunde T., Omorogie M.O., O'Brien P. 21. Ferromagnetic FeSe2 from a mixed sulphur-selenium complex of iron [Fe{(SePPh2NPPh2S)2N}3] through pyrolysis. Heliyon, 2020, vol. 6, Issue

4, e03763.

doi.org/10.1016/j.heliyon.2020.e03763 22.

16. Kong F., Zheng J., Tao Sh., Qian B. Electrochemical and electrocatalytic performance of FeSe2 nanoparticles improved by selenium matrix. Materials Letters, 2021, vol. 284, Part 2, p. 128947. 23.

17. Thanikaikarasan S., Perumal R., Marjorie R.S. Influence of potential on structural, compositional, optical and magnetic

properties of electrochemically grown iron selenide thin films. Journal of Alloys and Compounds, 2020, vol. 848, no. 25, p. 156348.

Tiana X.-H., Zhang J.-M. The structural, elastic, electronic and optical properties of orthorhombic FeX2 (X=S, Se, Te). Superlattices and Microstructures, 2018, vol. 119, pp. 201-211.

Majidzade V.A., Mammadova S.P., Petkucheva E.S., Slavcheva E.P., Aliyev A.Sh., Tagiyev D.B. Co-electrodeposition of iron and sulfur in aqueous and non-aqueous electrolytes. Bulgarian Chemical Communications, 2020, vol. 52, Special Issue E, pp. 73-78.

Majidzade V.A., Aliyev A.Sh. Electrodeposition of Ni3Bi2Se2 Thin Semiconductor Films._Iranian Journal of Chemistry and Chemical Engineering, 2021, vol. 40, Issue 4, serial Number 108, pp. 1023-1029.

Majidzade V.A., Aliyev A.Sh., Guliyev P.H., Babanly D.M. Electrodeposition of the Sb2Se3 thin films on various substrates from the tartaric electrolyte. Journal of Electrochemical Science and Engineering, 2020, vol. 10, no. 1, pp. 1-9. Aliyev A.Sh., Majidzade V.A., Soltanova N.Sh., Tagiyev D.B., Fateev V.N. Some features of electrochemically deposited CdS nanowires. Chemical Problems, 2018, no. 2, pp. 178-185.

Handbook of Electrochemistry, Sukhotin A.M. (Ed.), Leningrad: Khimiya Publ., 1981. p.488. (In Russian).

D9MiR MONOSELENiD NAZiK T9B9Q9L9RlNiN ELEKTROKiMY9Vi SiNTEZi

A.O. Zeynalova, S.P. Cavadova, V.A. Macidzada, A.§. 91iyev

AMEA akad. M. Nagiyev adina Kataliz vd Qeyri-uzvi kimya institutu AZ1143, H. Cavidprospekti 113, Baki, Azdrbaycan, e-mail: vuska_80@mail.ru

Taqdim olunan i§da damir, selen va Fe-Se nazik tabalarinin 9okma prosesinin kinetika va mexanizmi Pt va Ni elektrodlarindan istifada etmakla potensiodinamik usulla tsiklik va xatti polyarizasiya ayrilari 9akmakla oyranilmi§dir. Elektroliti ta§kil edan komponentlarin Pt elektrodu uzarinda ham ayriliqda, ham da birga 9okma potensial sahalari muayyan edilmi§dir. Optimal

elektroliz rejimi va elektrolit tarkibini müayyanla§dirmak ü9ün Fe-Se-nin birga elektro9ökma prosesina müxtalif amillarin (ilkin komponentlarin qatiligi, temperatur va s.) tasiri ara§dirilmi§dir. Bundan alava, Ni elektrodlari sathina 9ökdürülmü§ Fe-Se nümunalari 450oC temperaturda termiki emal olunmu§, SEM va rentgen-faza analiz metodlarii ila tadqiq edilmi§dir. Tabaqalarin element analizi göstarir ki, onlarin tarkibinda 42.2 % Fe va 57.8 % Se var. Keywords: elektrogökma, ddmir monoselenid, polyarizasiya, elektroreduksiya

ЭЛЕКТРОХИМИЧЕСКИЙ СИНТЕЗ ТОНКИХ ПЛЕНОК МОНОСЕЛЕНИДА

ЖЕЛЕЗА

А.О. Зейналова, С.П. Джавадова, В.А. Маджидзаде, А.Ш. Алиев

Институт Катализа и Неорганической Химии им. М. Нагиева НАНА AZ1143, пр. Г. Джавида 113, Баку, Азербайджан vuska_80@mail.ru

В представленной работе исследованы кинетика и механизм процесса осаждения железа, селена и тонких пленок Fe-Se снятием циклических и линейных поляризационных кривых потенциодинамическим методом с использованием Pt и Ni электродов. Определены область потенциалов компонентов в отдельности в составе электролита и совместного их осаждения на Pt-электроде. С целью определения оптимального режима электролиза и состава электролита было изучено влияние различных факторов (концентрация исходных компонентов, температура и др.) на процесс электроосаждения Fe-Se. Кроме того, образцы Fe-Se, нанесенные на поверхность никелевых электродов, были подвергнуты термообработке при 4500С и исследованы методами СЭМ и рентгенофазового анализа. Элементный анализ пленок показывает, что они содержат 42.2 % Fe и 57.8 % Se.

Ключевые слова: электроосаждение, моноселенид железа, поляризация, электровосстановление.