- Email: [email protected]
MOCVD of Pt coatings with high surface areas on the contacts of electrophysiological diagnostic electrodes
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 19 (2019) 924–930
www.materialstoday.com/proceedings
BraMat 2019
MOCVD of Pt coatings with high surface areas on the contacts of electrophysiological diagnostic electrodes Svetalana I Dorovskikha,b,*, Danil B Kal'nuia,b, Eugenii.V. Maximovskyia,b, Natalia B. Morozovaa a
Nikolaev Institute of Inorganic Chemistry, SB RAS, Lavrentieva Ave. 3, 630090, Novosibirsk, Russia b Novosibirsk State University, Pirogova Str. 2, Novosibirsk, 630090, Russia
Abstract The potential of a chemical vapor deposition technique to produce Pt coatings with high surface area contacts of an electrophysiological diagnostic electrode has been firstly demonstrated. On the first step, a Pd layer has been deposited in the hydrogen atmosphere from Pd(hfac)2 on the contacts made of stainless steel. On the second step, the top functional Pt coatings have been deposited in the oxygen atmosphere from Pt(acac)2. The effect of deposition parameters and the Pd sublayer on the microstructural and electrochemical characteristics of Pt coatings has been studied. The capacity and impedance values of samples are 22–198 mC·cm–2 (0.1M H2SO4, 100 mV/s) and 56–167 Ω (at 120 Hz), respectively. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 11th International Conference on Materials Science & Engineering, BraMat 2019 Keywords: Pt coatings; Pd sublayer; contact of an EFI electrode; CV.
1. Introduction An electrophysiological investigation (EPI) of a heart is the main procedure used for diagnostics and treatment of cardiovascular diseases. The EPI procedure is based on the implantation of diagnostic multi-pole (from 2 to 20 contacts) electrodes (Fig. 1) used to measure electrical potentials in various areas of a heart, intervals and rate of impulses, localization of pathological areas, and ways of conducting cardiac pulses [1].
* Corresponding author Tel: +7(383)3309556, Fax: +7(838) 3309489 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 11th International Conference on Materials Science & Engineering, BraMat 2019
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
925
Nomenclature MOCVD Pd(hfac)2 Pt(acac)2 CSC
metalorganic chemical vapor deposition bis(1,1,1,5,5,5-hexafluoro-pentane-2,4-dionato)palladium bis(pentane-2,4-dionato)platinum Charge storage capacity
Fig. 1. The contact of an EFI electrode.
The correlation between an active surface area of an electrode and its functional characteristics (efficiency of the electrode stimulation and lifetime) was firstly established by Schaldach [2]. The main idea is based on increasing a surface area of a medical electrode by depositing biocompatible functional fractal-like coatings on its surface. Platinum, combining biocompatibility, corrosion resistance, thermal conductivity and required electrochemical characteristics, is one of the most demanded materials for medical electrodes. Being a radiopaque material, Pt-based electrodes [3, 4] facilitated the visualization during diagnostic procedures or operations. The problem of obtaining a noble metal coating with the required morphology on medical electrodes is discussed in [5-7]. Petrossians et al. [5] and Ganske et al. [6] succeed in depositing Pt-containing coatings on planar electrodes as Si substrates using electrodeposition and magnetron sputtering, respectively. There are few examples of depositing noble metals on non-planar electrodes. Andreev et al. [7] produced a thick (~ 50 µm) noble metal layer using the physical vapor deposition technique. Recently, Vikulova et al. [8] and Gelfond et al. [9] succeed in depositing fractal Ir and Pt coatings on pole tips of the endocardial electrode using a chemical vapor deposition (MOCVD) technique. In general, the MOCVD technique seems to be a promising method to produce functional coatings on the contacts of an EFI electrode, taking into account the features of contact materials (stainless steel or polymers) and the nonplanar geometry of contacts. In the present work, the MOCVD technique has been firstly used to deposit Pt coatings on the surface of EFI electrode contacts. Hence, the strategy of using a Pd layer to protect the contact material from degradation has been proposed for the first time. The effect of the Pd sublayer and deposition parameters on a microstructure of functional Pt coatings has been studied. The electrochemical characteristics of coatings have been measured to demonstrate their suitability to apply on an EFI electrode. 2. Experimental The available metal beta-diketonates, namely Pd(hfac)2 and Pt(acac)2 were used as MOCVD precursors for the repetitive deposition of Pd and Pt coatings. The elemental analysis was carried out on a Carlo-Erba 1106 instrument. Anal. Calc. for Pd(hfac): C, 27.1; H, 0.4%; F, 43.8%. Found: C, 27.0; H, 0.2% F, 44.0%; anal. calc. for Pt(acac)2: C, 30.7; H, 3.50%, found: C, 31.1; H, 3.8%.
926
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
The stainless steel contacts of an EFI electrode (Fig. 1) were purchased from Co Ltd “Elestim-Cardio” (Russia). Due to the non-planarity of contacts it is rather difficult to determine the composition and thickness of coatings deposited on them. For this reason, planar Si(100) plates (1 x 1 cm2) were used as model substrates in the same experiments. The Pd layers were deposited at a total pressure of 2 Torr in the hydrogen atmosphere (v(H2) 1 L•h-1). The vaporization temperature (Tvap.) estimated from the p-T dependence [11] was 50ºC, while the gas-carrier flow rate was (v(Ar) 5 L•h-1). The substrate temperature (Ts.) of 200ºC was chosen to deposit samples with a compact structure and a minimum impurity level [13]; time of experiments was 10-120 min. The growth conditions for Pt coatings were selected to provide high growth rates of Pt samples, based on the p-T dependence [12], as follows: Tvap. was 165°С; v(Ar) – 6 L•h-1; v(O2) – 3 L•h-1; total pressure was 50 Torr; time of experiments was 180 min (Table 1). The Ts. range was 260-300°C, according to the previous results [9]. The X-ray diffraction (XRD) analysis of the samples was performed on a Shimadzu XRD-7000 diffractometer (CuKα radiation, Ni filter, step-by-step mode, angular range 2θ = 25–60°). X-ray photoelectron spectra (XPS) of the planar samples were recorded using a Phoibos-150 SPECS spectrometer with AlKα monochromatized X-ray radiation. The transmission energy of the electron analyzer was 20 eV. The binding energy was measured relative to Pt4f7/2 = 71.2 eV [13]. The in-depth measurements of the sample composition were carried out by Ar+ ion beam etching at 1 keV; the sample current was ~ 7 μA/cm2; the estimated etching rate was ~ 0.5 nm/min. The elemental composition, surface morphology, and microstructure of the samples were investigated using the following instruments: JSM-6700F connected with an EX-2300BU analyzer. The thickness of coatings deposited on Si(100) substrates was evaluated from the cross-section SEM data. The CSC values of Pt/Pd/contact samples were calculated from the cyclic voltammetry data by integrating the cathode (CSCC) and anode (CSCA) fields of CV curves and were normalized to the geometric surface of the contacts (0.2 cm2). The electrochemical experiments were carried out on the P-30J potentiostat-galvanostat in a range of the working electrode potential (studied sample) -250-1250 mV (0.1M H2SO4 solution, scan rate 100 mV·s-1). The roughness factor (Rf) for the Pt samples was calculated as the ratio of the electrochemically determined surface area of Pt samples to their geometric surface area. The impedance values of Pt/Pd/contact samples were measured using the Z-1500J impedancemeter at a frequency of 10–2-105 Hz, amplitude 10 mV, at the open circuit potential in a 0.9% NaCl solution. The reference electrode in the two-electrode electrochemical cell was a titanium plate (5.5 х 5.5 cm2). The working electrode was immersed to a depth of 6-8 cm. The distance between the working and reference electrodes was 8-10 cm. The measured electrochemical characteristics of the Pt/Pd samples are presented in Table. 1 Table 1. Deposition conditions and electrochemical characteristics of samples. Sample
1
Ts., °С
Substrate
260
2
Thicknesses of Pd layer on the Si
Cyclic Voltammetry
Impedance characteristics
CSC, mС/cm2
Rf*
C, 10-3F/cm2
R, Ω
Pd/Si
160 nm (20 min)
-
-
-
-
Si, Pd/Si, Pd/contact
900 nm (120 min)
198
86
1.00
56
3
280
Si, Pd/Si, Pd/contact
900 nm(120 min)
109
35
0.70
97
4
300
Si, Pd/Si, Pd/contact
90 nm (10 min)
22
11
0.08
167
Si, Pd/Si, contact
900 nm (120 min)
24
13
0.10
86
5
3. Results and discussion Progress in designing medical electrodes is based on using the alternative type of electrode materials, such as cheap stainless steel or polymers instead of bulk titanium or platinum electrodes. One the other hand, it is actual to reduce the geometric dimensions (diameter / cross-section) of an electrode or its pieces with maintaining the electrophysical properties, the quality of the biosignal detection and the long-term reduction in stimulation
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
927
thresholds. Nowadays, the electrode surface modification via the deposition of functional noble metal coatings with high surface areas is one of the promising ways to achieve the optimal energy transfer at the «electrode-cardiacmuscle» interface [5-8]. Following this trend, this paper is aimed on depositing Pt coatings with high surface areas on EFI electrode contacts made of stainless steel by the MOCVD technique. According to our previous results [14], Pt films with high surface areas and a minimum impurity level can be deposited in the presence of oxygen, while the use of hydrogen led to the formation of Pt films with smooth surfaces. Due to the sensitivity of the contact material (stainless steel) to oxidation, the idea of using a Pd layer to protect the contact material as well as improve the adhesion of the top Pt layer has been proposed for the first time. In addition, the effect of a Pd sublayer on the microstructural features of the functional Pt layer has been investigated. According to the data [15], fluorine-free Pd layers can be deposited from Pd(hfac)2 in the hydrogen atmosphere in the wide temperature range Ts. = 80-200°C, which makes this system attractive for the low temperature deposition of a Pd layer on various types of materials, including polymers. Recently, Pd layers with a compact structure have been deposited on metal phthalocyanine in the temperature range Ts = 130-200°C. Taking into account the data published in [12, 16], the deposition conditions were selected and Pd layers with an average Pd content of 85 at.% were deposited on both Si and contact substrates. The Pd layers consisting on the spherical grains combined into agglomerates were grown on both type of substrates. The average size of agglomerates formed on the surface of contact samples is higher than that for Si samples that might arise from the higher growth rates in the first case. The thicknesses of Pd/Si samples expectedly increase from 160 to 900 nm with the time of experiments increasing from 20 to 120 min, respectively (Fig. 2a and 2b). On the initial step of the film growth, a thin compact layer formed. The further growth of the film is accompanied by the appearance of a layer with a dense structure on the top of the initial layer. Cavities observed in the structure of this layer are likely to be due to gas diffusion during the layer growth.
Fig. 2. (a) Cross-section SEM images of Pd layers deposited on Si at 20 and 120min, respectively.
Typical XRD patterns of samples deposited on the Pd/contact substrates are presented in Fig. 3a.
Fig. 3. (a) XRD patterns of samples deposited on Pd/contact and (b) XPS spectra of sample 2 before and after etching with Ar+ ions.
928
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
There are diffraction peaks at 2θ = 39.76(111), 46.24(200), 61.70(220) related to fcc-Pt. An increase in the Pd layer thicknesses from 90 to 900 nm leads to the appearance of shoulder peaks at 2θ = 40.25(111), 46.45(200) corresponding to fcc-Pd on the XRD patterns of the samples. The Pt sample deposited on the Pd/contact at 260°C has the 100 orientation, while the further increase in Ts. leads to the growth of samples consisting on chaotically orientated crystallites. It should be noted that most of the samples obtained on Pd/Si substrates have 111 orientation. The typical XPS spectra of the sample 2 surface before and after 5 min etching are given in Fig. 3b. The Pt state does not change (71.2 (Pt4f7/2), 74.6 (Pt4f5/2)) [13] during etching. The Pt content in the sample after etching reached 90 at.%, while C and O contents were 8.5 and 1.5at.%, respectively. The feather-like columnar structure typical of Pt samples deposited on Si [9] slightly changed if Pd/Si were used as substrates. Regardless of Pd sublayer microstructure (dense structure with cavities (Fig. 4a) either compact structure (Fig. 4b)), the structure of Pt samples deposited on Pd/Si was formed by chaotic orientated columns. The increase in Ts. from 260 to 300°C led to an increase in the average diameter of the columns. The thickness of Pt layers decreased from 1.1 to 0.43 µm with increasing Ts. from 260 to 300°C. Thus, the highest growth rate was reached at Ts.= 260°C.
Fig. 4. Cross-section SEM images of Pt samples deposited on Si: sample 1(a) and sample 4(b).
The surface of Pt coatings deposited on Pd/Si at temperatures up to 280°C consists of elongated grains with an average diameter no more than 200 nm (Fig. 5 a,b), while the pyramidal structures were grown under similar conditions on a Si substrate [9]. The change in the substrate type from Pd/Si to Pd/contact caused the growth of small grains (average diameter no more than 100 nm) which combined into agglomerates (Fig. 5 d,e).
Fig. 5. SEM images of Pt coatings deposited on Pd/Si: (a) sample 2; (b) sample 3; (с) sample 5; and corresponding Pt coatings deposited on Pd/contact: (d) sample 2; (e) sample 3; (f) sample 5.
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
929
These features of growth Pt/Pd/Si and Pt/Pd/contact samples may be due to differences of the crystallite orientation of samples. Regardless of substrate's type, the increase in Ts. from 260 to 300°C leads to the appearance of spherical agglomerates on the surface of samples (Fig.5 c, f) instead of elongated ones. The increase in Ts. above 300°C smoothes the sample surface and decreases its surface area. Indeed, Rf values calculated from the CV data to estimate the surface areas of the samples also decreases with increasing Ts. (Table 1). Typical CV profiles of some Pt/Pd/contact samples are shown in Fig. 6. The CV profiles of all samples are similar to those for the Pt electrode [17], which indicates the absence of corrosion processes. The cathode peaks observed in the negative potential range of CV curves of the samples are related to strong and weak adsorption of hydrogen. The peak observed in the cathode range at ~ 400 mV can be related to oxygen reduction that is typical of activated Pt electrodes [17]. The Rf and CSC values of the Pt/Pd/contact samples calculated from the CV data are compatible with those of the Pt samples deposited by MOCVD from (CH3)3Pt(acac)Py [14] and the samples produced by Ganske et al. [6], but are not superior to Pt samples deposited by MOCVD from Pt(acac)2 [9].
Fig. 6. CV curves of Pt samples deposited on Pd/contact recorded at a scan rate 100 mV/s.
The electrode impedance as one of the important functional characteristics affecting the life time of medical devices and advantages of using electrodes with low impedance values are widely discussed [18]. The impedance values of the studied samples were calculated from the spectra at 120 Hz. The majority of the samples have low impedance values up to 102Ω compatible with those for Pt-containing samples produced by Petrossians et al. [5]. It is noticeable that the thickness of a Pd sublayer affects only the impedance values (Table 1). One of the possible reasons is the oxidation of a Pd layer during the deposition of Pt coatings. Additionally, the Helmholtz capacity values per electrode area determined from the impedance spectra were 0.08-1.00·10-3·F/cm2 (at 120 Hz). 4. Conclusion The features of the MOCVD formation of Pt coatings on the contacts of EPI electrodes have been studied for the first time. Pd and Pt β-diketonates were used as precursors to deposit a sublayer and a functional coating in hydrogen and oxygen atmospheres, respectively. The possibility of a low temperature deposition of a Pd sublayer with a dense structure on the surface of a contact was demonstrated. The Pt coatings deposited on Pd layers exhibit the low carbon impurity level, columnar structure, and a thickness up to ~ 1.1 µm. The possibility to control the surface morphology of Pt coatings via changing the substrate temperature is studied and the coatings with the highest surface areas are deposited in the range of substrate temperatures 260-280ºC. The effect of a Pd sublayer on the microstructural and electrochemical characteristics of Pt coatings is determined and the tendency of decreasing impedance values with increasing thicknesses of the Pd layer is observed. The impedance values of the Pt samples are 56-167 Ω (120 Hz), while the capacitive values of the Pt samples evaluated from cyclic voltammetry are in the ranges 22-198 mC•cm–2 (0.1M H2SO4 solution, 100 mV/s).
930
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
Acknowledgements The work was supported by the Russian Science Foundation project №. 18-73-00052 "MOCVD of formation of platinum-containing coatings on elements of medical devices" References [1] E.R. Behr, P. Elliott, W.J. McKenna, Cardiac electrophys. Rev. 6(4) (2002) 482–486. [2] M. Schaldach, Prog. Biomed. Res. 5 (2000) 259–272. [3] A. Cowley, Platinum Metals Rev. 55 (2) (2011) 98–107. [4] A. Johnson, T. Shiraishi, in: N. Baltzer, T. Copponnex (Eds.), Precious Metals for Biomedical Applications, Elsevier., UK, 2014, pp. 37–49. [5] A. Petrossians, J.J. Whalen, J.D. Weiland, F. Mansfeld, Electrochem. Soc. 158(5) (2011) D269–D276. [6] G .Ganske, E.van. Slavcheva, A. Ooyen, W. Mokwa, U. Schnakenberg, Thin Solid Films. 519(11) (2011) 3965–3970. [7] E.S. Andreev, Yu.B. Vasilenko, B.A. Vershok, A.A. Zverev, O.I. Obrezkov, Thin-film coating of pole tips of endocardial electrodes of cardiostimulators and method of its obtaining, RU 2013134271/02, 2013. [8] E.S. Vikulova, D.B. Kal’nyi, Y.V. Shubin, V.V. Kokovkin, N.B. Morozova, A. Hassan, T.V. Basova, Appl. Surf. Sci. 425 (2017), 1052–1058. [9] N.V. Gelfond, V.V. Krisyk, S.I. Dorovskikh, D.B. Kal’nyi, E.A. Maksimovskii, Y.V. Shubin, N.B. Morozova, J. Struc. Chem., 56(6) (2015) 1215–1219. [10] G.I. Zharkova, I.K. Igumenov, N.M. Tyukalevskaya, Koord. Khim. 14 (1987), 67–74 [11] S.I. Dorovskikh, E.S. Vikulova, N.S. Nikolaeva, A.D. Shushanyan, R.G. Parkhomenko, N.B. Morozova, T.V. Basova, Sci. Advan. Mater., 9(7) (2017)1087–1092. [12] N.B. Morozova, G.I. Zharkova, P.P. Semyannikov, S.V. Sysoev, I.K. Igumenov, N.E. Fedotova, N. V. Gelfond, Proceedings EuroCVD 13, Glyfada, Greece, 3 (2000) 609–616. [13] da L.A. Silva, V.A. Alves, de S.C. Castro, J.F.C. Boodts, Colloids and Surfaces A: Physicochem. Eng. Aspects 170 (2000) 119–126. [14] S.I. Dorovskikh, G.I. Zharkova, A.E. Turgambaeva, V.V. Krisyuk, N.B. Morozova, Appl. Organomet.Chem., 31(7) (2017). e3654–e3659. [15] V. Bhaskaran, M.J. Hampden‐Smith, T.T. Kodas, Chem. Vapor Depos. 3(5) (1997) 281–286. [16] N.S. Nikolaeva, R.G. Parkhomenko, D.D. Klyamer, A.D. Shushanyan, I.P. Asanov, N.B. Morozova, T.V. Basova, Int. J. Hydrogen Energy, 42(47) (2017) 28640–28646. [17] W.G. Pell, A. Zolfaghari, B.E. Conway, J. Electroanal. Chem. 532(1-2) (2002) 13–23. [18] A. Norlin, J. Pan, C. Leygraf, Biomolecular Engineering 19 (2002) 67–/71.
ScienceDirect Materials Today: Proceedings 19 (2019) 924–930
www.materialstoday.com/proceedings
BraMat 2019
MOCVD of Pt coatings with high surface areas on the contacts of electrophysiological diagnostic electrodes Svetalana I Dorovskikha,b,*, Danil B Kal'nuia,b, Eugenii.V. Maximovskyia,b, Natalia B. Morozovaa a
Nikolaev Institute of Inorganic Chemistry, SB RAS, Lavrentieva Ave. 3, 630090, Novosibirsk, Russia b Novosibirsk State University, Pirogova Str. 2, Novosibirsk, 630090, Russia
Abstract The potential of a chemical vapor deposition technique to produce Pt coatings with high surface area contacts of an electrophysiological diagnostic electrode has been firstly demonstrated. On the first step, a Pd layer has been deposited in the hydrogen atmosphere from Pd(hfac)2 on the contacts made of stainless steel. On the second step, the top functional Pt coatings have been deposited in the oxygen atmosphere from Pt(acac)2. The effect of deposition parameters and the Pd sublayer on the microstructural and electrochemical characteristics of Pt coatings has been studied. The capacity and impedance values of samples are 22–198 mC·cm–2 (0.1M H2SO4, 100 mV/s) and 56–167 Ω (at 120 Hz), respectively. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 11th International Conference on Materials Science & Engineering, BraMat 2019 Keywords: Pt coatings; Pd sublayer; contact of an EFI electrode; CV.
1. Introduction An electrophysiological investigation (EPI) of a heart is the main procedure used for diagnostics and treatment of cardiovascular diseases. The EPI procedure is based on the implantation of diagnostic multi-pole (from 2 to 20 contacts) electrodes (Fig. 1) used to measure electrical potentials in various areas of a heart, intervals and rate of impulses, localization of pathological areas, and ways of conducting cardiac pulses [1].
* Corresponding author Tel: +7(383)3309556, Fax: +7(838) 3309489 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 11th International Conference on Materials Science & Engineering, BraMat 2019
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
925
Nomenclature MOCVD Pd(hfac)2 Pt(acac)2 CSC
metalorganic chemical vapor deposition bis(1,1,1,5,5,5-hexafluoro-pentane-2,4-dionato)palladium bis(pentane-2,4-dionato)platinum Charge storage capacity
Fig. 1. The contact of an EFI electrode.
The correlation between an active surface area of an electrode and its functional characteristics (efficiency of the electrode stimulation and lifetime) was firstly established by Schaldach [2]. The main idea is based on increasing a surface area of a medical electrode by depositing biocompatible functional fractal-like coatings on its surface. Platinum, combining biocompatibility, corrosion resistance, thermal conductivity and required electrochemical characteristics, is one of the most demanded materials for medical electrodes. Being a radiopaque material, Pt-based electrodes [3, 4] facilitated the visualization during diagnostic procedures or operations. The problem of obtaining a noble metal coating with the required morphology on medical electrodes is discussed in [5-7]. Petrossians et al. [5] and Ganske et al. [6] succeed in depositing Pt-containing coatings on planar electrodes as Si substrates using electrodeposition and magnetron sputtering, respectively. There are few examples of depositing noble metals on non-planar electrodes. Andreev et al. [7] produced a thick (~ 50 µm) noble metal layer using the physical vapor deposition technique. Recently, Vikulova et al. [8] and Gelfond et al. [9] succeed in depositing fractal Ir and Pt coatings on pole tips of the endocardial electrode using a chemical vapor deposition (MOCVD) technique. In general, the MOCVD technique seems to be a promising method to produce functional coatings on the contacts of an EFI electrode, taking into account the features of contact materials (stainless steel or polymers) and the nonplanar geometry of contacts. In the present work, the MOCVD technique has been firstly used to deposit Pt coatings on the surface of EFI electrode contacts. Hence, the strategy of using a Pd layer to protect the contact material from degradation has been proposed for the first time. The effect of the Pd sublayer and deposition parameters on a microstructure of functional Pt coatings has been studied. The electrochemical characteristics of coatings have been measured to demonstrate their suitability to apply on an EFI electrode. 2. Experimental The available metal beta-diketonates, namely Pd(hfac)2 and Pt(acac)2 were used as MOCVD precursors for the repetitive deposition of Pd and Pt coatings. The elemental analysis was carried out on a Carlo-Erba 1106 instrument. Anal. Calc. for Pd(hfac): C, 27.1; H, 0.4%; F, 43.8%. Found: C, 27.0; H, 0.2% F, 44.0%; anal. calc. for Pt(acac)2: C, 30.7; H, 3.50%, found: C, 31.1; H, 3.8%.
926
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
The stainless steel contacts of an EFI electrode (Fig. 1) were purchased from Co Ltd “Elestim-Cardio” (Russia). Due to the non-planarity of contacts it is rather difficult to determine the composition and thickness of coatings deposited on them. For this reason, planar Si(100) plates (1 x 1 cm2) were used as model substrates in the same experiments. The Pd layers were deposited at a total pressure of 2 Torr in the hydrogen atmosphere (v(H2) 1 L•h-1). The vaporization temperature (Tvap.) estimated from the p-T dependence [11] was 50ºC, while the gas-carrier flow rate was (v(Ar) 5 L•h-1). The substrate temperature (Ts.) of 200ºC was chosen to deposit samples with a compact structure and a minimum impurity level [13]; time of experiments was 10-120 min. The growth conditions for Pt coatings were selected to provide high growth rates of Pt samples, based on the p-T dependence [12], as follows: Tvap. was 165°С; v(Ar) – 6 L•h-1; v(O2) – 3 L•h-1; total pressure was 50 Torr; time of experiments was 180 min (Table 1). The Ts. range was 260-300°C, according to the previous results [9]. The X-ray diffraction (XRD) analysis of the samples was performed on a Shimadzu XRD-7000 diffractometer (CuKα radiation, Ni filter, step-by-step mode, angular range 2θ = 25–60°). X-ray photoelectron spectra (XPS) of the planar samples were recorded using a Phoibos-150 SPECS spectrometer with AlKα monochromatized X-ray radiation. The transmission energy of the electron analyzer was 20 eV. The binding energy was measured relative to Pt4f7/2 = 71.2 eV [13]. The in-depth measurements of the sample composition were carried out by Ar+ ion beam etching at 1 keV; the sample current was ~ 7 μA/cm2; the estimated etching rate was ~ 0.5 nm/min. The elemental composition, surface morphology, and microstructure of the samples were investigated using the following instruments: JSM-6700F connected with an EX-2300BU analyzer. The thickness of coatings deposited on Si(100) substrates was evaluated from the cross-section SEM data. The CSC values of Pt/Pd/contact samples were calculated from the cyclic voltammetry data by integrating the cathode (CSCC) and anode (CSCA) fields of CV curves and were normalized to the geometric surface of the contacts (0.2 cm2). The electrochemical experiments were carried out on the P-30J potentiostat-galvanostat in a range of the working electrode potential (studied sample) -250-1250 mV (0.1M H2SO4 solution, scan rate 100 mV·s-1). The roughness factor (Rf) for the Pt samples was calculated as the ratio of the electrochemically determined surface area of Pt samples to their geometric surface area. The impedance values of Pt/Pd/contact samples were measured using the Z-1500J impedancemeter at a frequency of 10–2-105 Hz, amplitude 10 mV, at the open circuit potential in a 0.9% NaCl solution. The reference electrode in the two-electrode electrochemical cell was a titanium plate (5.5 х 5.5 cm2). The working electrode was immersed to a depth of 6-8 cm. The distance between the working and reference electrodes was 8-10 cm. The measured electrochemical characteristics of the Pt/Pd samples are presented in Table. 1 Table 1. Deposition conditions and electrochemical characteristics of samples. Sample
1
Ts., °С
Substrate
260
2
Thicknesses of Pd layer on the Si
Cyclic Voltammetry
Impedance characteristics
CSC, mС/cm2
Rf*
C, 10-3F/cm2
R, Ω
Pd/Si
160 nm (20 min)
-
-
-
-
Si, Pd/Si, Pd/contact
900 nm (120 min)
198
86
1.00
56
3
280
Si, Pd/Si, Pd/contact
900 nm(120 min)
109
35
0.70
97
4
300
Si, Pd/Si, Pd/contact
90 nm (10 min)
22
11
0.08
167
Si, Pd/Si, contact
900 nm (120 min)
24
13
0.10
86
5
3. Results and discussion Progress in designing medical electrodes is based on using the alternative type of electrode materials, such as cheap stainless steel or polymers instead of bulk titanium or platinum electrodes. One the other hand, it is actual to reduce the geometric dimensions (diameter / cross-section) of an electrode or its pieces with maintaining the electrophysical properties, the quality of the biosignal detection and the long-term reduction in stimulation
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
927
thresholds. Nowadays, the electrode surface modification via the deposition of functional noble metal coatings with high surface areas is one of the promising ways to achieve the optimal energy transfer at the «electrode-cardiacmuscle» interface [5-8]. Following this trend, this paper is aimed on depositing Pt coatings with high surface areas on EFI electrode contacts made of stainless steel by the MOCVD technique. According to our previous results [14], Pt films with high surface areas and a minimum impurity level can be deposited in the presence of oxygen, while the use of hydrogen led to the formation of Pt films with smooth surfaces. Due to the sensitivity of the contact material (stainless steel) to oxidation, the idea of using a Pd layer to protect the contact material as well as improve the adhesion of the top Pt layer has been proposed for the first time. In addition, the effect of a Pd sublayer on the microstructural features of the functional Pt layer has been investigated. According to the data [15], fluorine-free Pd layers can be deposited from Pd(hfac)2 in the hydrogen atmosphere in the wide temperature range Ts. = 80-200°C, which makes this system attractive for the low temperature deposition of a Pd layer on various types of materials, including polymers. Recently, Pd layers with a compact structure have been deposited on metal phthalocyanine in the temperature range Ts = 130-200°C. Taking into account the data published in [12, 16], the deposition conditions were selected and Pd layers with an average Pd content of 85 at.% were deposited on both Si and contact substrates. The Pd layers consisting on the spherical grains combined into agglomerates were grown on both type of substrates. The average size of agglomerates formed on the surface of contact samples is higher than that for Si samples that might arise from the higher growth rates in the first case. The thicknesses of Pd/Si samples expectedly increase from 160 to 900 nm with the time of experiments increasing from 20 to 120 min, respectively (Fig. 2a and 2b). On the initial step of the film growth, a thin compact layer formed. The further growth of the film is accompanied by the appearance of a layer with a dense structure on the top of the initial layer. Cavities observed in the structure of this layer are likely to be due to gas diffusion during the layer growth.
Fig. 2. (a) Cross-section SEM images of Pd layers deposited on Si at 20 and 120min, respectively.
Typical XRD patterns of samples deposited on the Pd/contact substrates are presented in Fig. 3a.
Fig. 3. (a) XRD patterns of samples deposited on Pd/contact and (b) XPS spectra of sample 2 before and after etching with Ar+ ions.
928
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
There are diffraction peaks at 2θ = 39.76(111), 46.24(200), 61.70(220) related to fcc-Pt. An increase in the Pd layer thicknesses from 90 to 900 nm leads to the appearance of shoulder peaks at 2θ = 40.25(111), 46.45(200) corresponding to fcc-Pd on the XRD patterns of the samples. The Pt sample deposited on the Pd/contact at 260°C has the 100 orientation, while the further increase in Ts. leads to the growth of samples consisting on chaotically orientated crystallites. It should be noted that most of the samples obtained on Pd/Si substrates have 111 orientation. The typical XPS spectra of the sample 2 surface before and after 5 min etching are given in Fig. 3b. The Pt state does not change (71.2 (Pt4f7/2), 74.6 (Pt4f5/2)) [13] during etching. The Pt content in the sample after etching reached 90 at.%, while C and O contents were 8.5 and 1.5at.%, respectively. The feather-like columnar structure typical of Pt samples deposited on Si [9] slightly changed if Pd/Si were used as substrates. Regardless of Pd sublayer microstructure (dense structure with cavities (Fig. 4a) either compact structure (Fig. 4b)), the structure of Pt samples deposited on Pd/Si was formed by chaotic orientated columns. The increase in Ts. from 260 to 300°C led to an increase in the average diameter of the columns. The thickness of Pt layers decreased from 1.1 to 0.43 µm with increasing Ts. from 260 to 300°C. Thus, the highest growth rate was reached at Ts.= 260°C.
Fig. 4. Cross-section SEM images of Pt samples deposited on Si: sample 1(a) and sample 4(b).
The surface of Pt coatings deposited on Pd/Si at temperatures up to 280°C consists of elongated grains with an average diameter no more than 200 nm (Fig. 5 a,b), while the pyramidal structures were grown under similar conditions on a Si substrate [9]. The change in the substrate type from Pd/Si to Pd/contact caused the growth of small grains (average diameter no more than 100 nm) which combined into agglomerates (Fig. 5 d,e).
Fig. 5. SEM images of Pt coatings deposited on Pd/Si: (a) sample 2; (b) sample 3; (с) sample 5; and corresponding Pt coatings deposited on Pd/contact: (d) sample 2; (e) sample 3; (f) sample 5.
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
929
These features of growth Pt/Pd/Si and Pt/Pd/contact samples may be due to differences of the crystallite orientation of samples. Regardless of substrate's type, the increase in Ts. from 260 to 300°C leads to the appearance of spherical agglomerates on the surface of samples (Fig.5 c, f) instead of elongated ones. The increase in Ts. above 300°C smoothes the sample surface and decreases its surface area. Indeed, Rf values calculated from the CV data to estimate the surface areas of the samples also decreases with increasing Ts. (Table 1). Typical CV profiles of some Pt/Pd/contact samples are shown in Fig. 6. The CV profiles of all samples are similar to those for the Pt electrode [17], which indicates the absence of corrosion processes. The cathode peaks observed in the negative potential range of CV curves of the samples are related to strong and weak adsorption of hydrogen. The peak observed in the cathode range at ~ 400 mV can be related to oxygen reduction that is typical of activated Pt electrodes [17]. The Rf and CSC values of the Pt/Pd/contact samples calculated from the CV data are compatible with those of the Pt samples deposited by MOCVD from (CH3)3Pt(acac)Py [14] and the samples produced by Ganske et al. [6], but are not superior to Pt samples deposited by MOCVD from Pt(acac)2 [9].
Fig. 6. CV curves of Pt samples deposited on Pd/contact recorded at a scan rate 100 mV/s.
The electrode impedance as one of the important functional characteristics affecting the life time of medical devices and advantages of using electrodes with low impedance values are widely discussed [18]. The impedance values of the studied samples were calculated from the spectra at 120 Hz. The majority of the samples have low impedance values up to 102Ω compatible with those for Pt-containing samples produced by Petrossians et al. [5]. It is noticeable that the thickness of a Pd sublayer affects only the impedance values (Table 1). One of the possible reasons is the oxidation of a Pd layer during the deposition of Pt coatings. Additionally, the Helmholtz capacity values per electrode area determined from the impedance spectra were 0.08-1.00·10-3·F/cm2 (at 120 Hz). 4. Conclusion The features of the MOCVD formation of Pt coatings on the contacts of EPI electrodes have been studied for the first time. Pd and Pt β-diketonates were used as precursors to deposit a sublayer and a functional coating in hydrogen and oxygen atmospheres, respectively. The possibility of a low temperature deposition of a Pd sublayer with a dense structure on the surface of a contact was demonstrated. The Pt coatings deposited on Pd layers exhibit the low carbon impurity level, columnar structure, and a thickness up to ~ 1.1 µm. The possibility to control the surface morphology of Pt coatings via changing the substrate temperature is studied and the coatings with the highest surface areas are deposited in the range of substrate temperatures 260-280ºC. The effect of a Pd sublayer on the microstructural and electrochemical characteristics of Pt coatings is determined and the tendency of decreasing impedance values with increasing thicknesses of the Pd layer is observed. The impedance values of the Pt samples are 56-167 Ω (120 Hz), while the capacitive values of the Pt samples evaluated from cyclic voltammetry are in the ranges 22-198 mC•cm–2 (0.1M H2SO4 solution, 100 mV/s).
930
S.I. Dorovskikh et al. / Materials Today: Proceedings 19 (2019) 924–930
Acknowledgements The work was supported by the Russian Science Foundation project №. 18-73-00052 "MOCVD of formation of platinum-containing coatings on elements of medical devices" References [1] E.R. Behr, P. Elliott, W.J. McKenna, Cardiac electrophys. Rev. 6(4) (2002) 482–486. [2] M. Schaldach, Prog. Biomed. Res. 5 (2000) 259–272. [3] A. Cowley, Platinum Metals Rev. 55 (2) (2011) 98–107. [4] A. Johnson, T. Shiraishi, in: N. Baltzer, T. Copponnex (Eds.), Precious Metals for Biomedical Applications, Elsevier., UK, 2014, pp. 37–49. [5] A. Petrossians, J.J. Whalen, J.D. Weiland, F. Mansfeld, Electrochem. Soc. 158(5) (2011) D269–D276. [6] G .Ganske, E.van. Slavcheva, A. Ooyen, W. Mokwa, U. Schnakenberg, Thin Solid Films. 519(11) (2011) 3965–3970. [7] E.S. Andreev, Yu.B. Vasilenko, B.A. Vershok, A.A. Zverev, O.I. Obrezkov, Thin-film coating of pole tips of endocardial electrodes of cardiostimulators and method of its obtaining, RU 2013134271/02, 2013. [8] E.S. Vikulova, D.B. Kal’nyi, Y.V. Shubin, V.V. Kokovkin, N.B. Morozova, A. Hassan, T.V. Basova, Appl. Surf. Sci. 425 (2017), 1052–1058. [9] N.V. Gelfond, V.V. Krisyk, S.I. Dorovskikh, D.B. Kal’nyi, E.A. Maksimovskii, Y.V. Shubin, N.B. Morozova, J. Struc. Chem., 56(6) (2015) 1215–1219. [10] G.I. Zharkova, I.K. Igumenov, N.M. Tyukalevskaya, Koord. Khim. 14 (1987), 67–74 [11] S.I. Dorovskikh, E.S. Vikulova, N.S. Nikolaeva, A.D. Shushanyan, R.G. Parkhomenko, N.B. Morozova, T.V. Basova, Sci. Advan. Mater., 9(7) (2017)1087–1092. [12] N.B. Morozova, G.I. Zharkova, P.P. Semyannikov, S.V. Sysoev, I.K. Igumenov, N.E. Fedotova, N. V. Gelfond, Proceedings EuroCVD 13, Glyfada, Greece, 3 (2000) 609–616. [13] da L.A. Silva, V.A. Alves, de S.C. Castro, J.F.C. Boodts, Colloids and Surfaces A: Physicochem. Eng. Aspects 170 (2000) 119–126. [14] S.I. Dorovskikh, G.I. Zharkova, A.E. Turgambaeva, V.V. Krisyuk, N.B. Morozova, Appl. Organomet.Chem., 31(7) (2017). e3654–e3659. [15] V. Bhaskaran, M.J. Hampden‐Smith, T.T. Kodas, Chem. Vapor Depos. 3(5) (1997) 281–286. [16] N.S. Nikolaeva, R.G. Parkhomenko, D.D. Klyamer, A.D. Shushanyan, I.P. Asanov, N.B. Morozova, T.V. Basova, Int. J. Hydrogen Energy, 42(47) (2017) 28640–28646. [17] W.G. Pell, A. Zolfaghari, B.E. Conway, J. Electroanal. Chem. 532(1-2) (2002) 13–23. [18] A. Norlin, J. Pan, C. Leygraf, Biomolecular Engineering 19 (2002) 67–/71.