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Electrochemical properties of stoichiometric RuN film prepared by rf-magnetron sputtering: A preliminary study
Electrochemistry Communications 49 (2014) 9–13
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrochemical properties of stoichiometric RuN film prepared by rf-magnetron sputtering: A preliminary study Davide Rosestolato a,⁎, Giancarlo Battaglin b, Sergio Ferro a a b
Dipartimento di Scienze Chimiche e Farmaceutiche, Università degli Studi di Ferrara, via Fossato di Mortara 17, 44121 Ferrara, Italy Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca' Foscari Venezia, Via Torino 155/B, 30170 Mestre— VE, Italy
a r t i c l e
i n f o
Article history: Received 2 September 2014 Received in revised form 18 September 2014 Accepted 29 September 2014 Available online 5 October 2014 Keywords: Ruthenium mononitride Rf-magnetron sputtering Hydrogen evolution reaction Thin film stability Adsorbed intermediates
a b s t r a c t The electrochemical properties and stability of ruthenium mononitride (RuN) thin films were studied. Coatings of RuN were synthesized on electropolished titanium supports by rf-magnetron sputtering. RuN electrodes appear rather stable against dissolution, independently of pH, but show to possess the greatest stability only in alkaline environment. Under hydrogen evolution conditions the films show relevant catalytic properties, comparable with Pt, Pd and Ru/Ir derivatives; a significant coverage by adsorbed reaction intermediates is involved. These electrodes are of potential application in energetics and sensoring. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The persistent and growing interest for materials, to be used in catalysis and energy storage applications, has prompted the research towards the study of platinum group metal nitrides. Their structural and morphological properties, combined with electronic features [1–3], make them attractive for different purposes. The features of nitrides and carbides of noble metals have been reviewed by Ivanovskii [4] in 2009. Since these compounds are generally characterized by a positive enthalpy of formation, accompanied by sensitivity to high temperatures [5], the reactive sputtering synthesis appears suitable for obtaining films with a well-defined stoichiometry. In this context, the present study has been focused on ruthenium nitride. Previous works have shown that stoichiometric RuN can be synthesized by pulsed laser ablation [6] and dc-magnetron sputtering [7]; the possibility to exploit also the rf-magnetron sputtering has been highlighted only recently [8]. In this paper, an electrochemical characterization of RuN films was carried out by means of cyclic voltammetry, polarization curves and impedance spectroscopy, under hydrogen evolution conditions, with the aim of obtaining information regarding the stability and catalytic activity of this material, which could be used as a cathode in different applications.
RuN films were synthesized by radio-frequency reactive sputtering, at room temperature, using the optimized procedure described in Ref. [8]. Samples were deposited both on electropolished (following the procedure described in Ref. [9]) titanium supports (2 × 2 cm), as well as onto silica sheets (1 × 1 cm). The thickness of obtained films was measured with a Tencor alpha-step 500 profilometer, on a step obtained by covering a small portion of the substrate during film deposition. The film composition was analyzed by Rutherford Backscattering Spectroscopy (RBS) at Laboratori Nazionali INFN-Legnaro (Padova, Italy) using a 2 MeV 4He+ beam with a spot size of about 2 mm2. Depth profiles were modeled by means of the RUMP computer code [10]. X-ray diffraction (XRD) experiments were accomplished with a Bruker D8 equipped with a Cu Kα source; a step size of 0.02° was adopted. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX) characterizations were performed with a FEIQUANTA 400 at the electron microscopy center of the University of Ferrara (Ferrara, Italy). In order to carry out the electrochemical investigation, a triple-body cell has been adopted, whose geometry has been described e.g. in Ref. [11]. All chemicals used were of high purity degree.
3. Results and discussion
⁎ Corresponding author. Tel.: +39 0532 455166. E-mail address: [email protected] (D. Rosestolato).
http://dx.doi.org/10.1016/j.elecom.2014.09.019 1388-2481/© 2014 Elsevier B.V. All rights reserved.
The analysis of the RBS spectrum shown in Fig. 1A confirms that prepared coatings possess a Ru:N stoichiometry very close to 1:1, while measurements with the profilometer evidenced a thickness of
10
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
Fig. 1. (A) RBS, (B) XRD and (C) SEM analyses for RuN sample synthesized on titanium; in (A), experimental points are shown as dots, while the continuous line is the result of RUMP fitting.
0.55 μm. Based on signal profiles, for which no concentration gradient can be recognized, a quite homogeneous film growth is evident. In agreement with previous literature data [6–8], the XRD investigation (Fig. 1B) indicated that a cubic crystalline phase has been obtained. The so-called face-centered cubic (fcc) packing is compatible with a number of structures: those of sodium chloride, anti-fluorite and zincblende are generally taken into account [6]. Of course, the charge density characteristic of considered species justifies different volumes for the lattice cells. In Fig. 1B, an important asymmetric peak is evident at 2ϑ ≈ 34°, which is plausibly due to the convolution of signals from Ti (support) and from the {111} planes of RuN cubic lattice. A number of less intense signals can be further recognized: peaks corresponding to the {200} and {220} reflective planes appear at 2ϑ ≈ 40° and 58°, respectively; eventually, the {311} family plane appear at about 70°. Focusing on the peak associated to the {220} family plane, which falls at about 57.93° in a region free from other signals, the calculation of the a = b = c cell parameter gave a value of 0.450 nm, typical of the zinc-blende structure. This outcome is in good agreement with evidences obtained in a previous work [8]. Moreover, based on the Scherrer equation:
t¼
λ FWHM cos ϑ
the average crystallite size can be estimated to be about 18 nm. SEM analyses (Fig. 1C) show a rough surface, well organized in pseudo-pyramidal structures having a triangular base. A sub-micro or even nano-porosity is evidently present along the whole material surface, with possible effects on its electrochemical properties.
As usual, the electrochemical investigation was started by collecting some preliminary CV curves; samples were preliminarily conditioned, by immersion in 1 M NaClO4 solution for 24 h. Since it has been reported that RuN is not stable in alkaline and acidic media [6], a neutral environment has been initially considered, limiting the window of polarization to the interval of water stability. As shown in Fig. 2A (CV in 1 M NaClO4), the electrode material seems to gradually change, reaching a stabilization after 400 scans. Its resistivity seems to decrease, and the shape of the CV curve becomes progressively similar to that of RuO2. The õelectrode was then investigated in 1 M HClO4 solution: after 400 scans, the appearance of broad signals characteristic of Ru(II)–Ru(III) and Ru(III)–Ru(IV) oxidation state transitions is evident (Fig. 2B). However, returning to the neutral environment, the response of the electrode turned out to be the same as previously obtained after the first 400 cycles. EDX analyses, performed on the “as deposited” and on the “electrochemical investigated” parts of the same sample, qualitatively indicate for the latter an increase in the oxygen content at the expense of the nitrogen one, thus confirming the hypothesis put forward on the basis of the voltammetric data. Since the electrode oxidation occurred without morphological modifications (the SEM analysis did not reveal any difference) the persistence of the characteristic columnar pyramidalterminated structure can be taken as an indication that oxidation occurred only in a thin, near surface region, of the RuN zinc-blend structure. After etching the first layers of the surface with an Ar beam, a composition close to that of the as prepared film was obtained, thus proving that the oxidation involves only the most external surface. Heavier changes (towards the more stable thermodynamic form) can probably be obtained by extending the electrochemical polarization for longer times.
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
11
Fig. 2. CV curves in different electrolytes and at different scan rates: (A) in 1 M NaClO4 at 100 mV s−1; (B) 400 cycles in 1 M solutions at 100 mV s−1; (C) in 1 M NaOH at 50 mV s−1. Indications in chart legends are reported in chronological order.
A new sample, prepared during the same round of deposition, was then studied, limiting the window of applied potential (from − 0.2 to 0.2 V vs. SCE) in order to avoid the modifications that seem to lead to the formation of the oxide. In this second turn of experiments, the scan rate was also reduced to 50 mV s−1, in order not to introduce kinetic constrictions that could affect the possible modification of the electrode surface. As shown in Fig. 2C, the material response reached a good stability after 400 cycles. Two aspects deserve to be underlined: the sample appears stable to dissolution, over the whole pH interval, and the alkaline environment seems to favor the stabilization of the external surface of the nitride. A confirmation was obtained by both EDX and RBS, which proved that the stoichiometry of the nitride surface did not change; as a result, the previous conditions (applied potential, and acidic pH) were responsible for the oxidation. Moreover, samples were left in the solutions for several hours, without showing any significant modification in composition, structure and thickness: this allows excluding any sort of corrosion/dissolution or modification of material, due to the chemical action of electrolytes. As a further way to characterize the material, the hydrogen evolution reaction (HER) was investigated: polarization curves were collected under steady-state regime in 1 M NaOH; in order to obtain reproducible data, the solution nearby the electrode surface was preliminarily enriched in dissolved hydrogen, by polarizing the electrode at a current density of 1000 A m−2 for 30 min. The steady-state polarization was performed step by step, moving from −1.24 V to −1.07 V vs. SCE with steps of 5 mV, and maintaining each polarization for 5 min before registering the current value. An example of obtained Tafel plots, corrected for the ohmic drop of the solution, is shown in Fig. 3A. A rather good linearity has been detected, which extended for a decade of current; however, the quite high value of Tafel slope is in disagreement with the conventional Volmer–Tafel and Volmer–Heyrovsky mechanisms, commonly found on other electrode materials [12]. The
exchange current density, estimated by extrapolating the high-field approximation line, resulted to be around 0.14 mA cm−2; in order to normalize the latter to the real surface, SEM images shown in Ref. [8] were considered, assuming that the film is formed by columnar pyramidal structures. Based on an approximated roughness factor of 1.15, an exchange current density of ≈0.12 mA cm−2real is obtained, which is comparable to values collected in acidic media at Pt, Pd, Rh and Ir [13], as well as in alkaline media at Pt electrodes (Ref. [14] and references therein). Electrochemical Impedance Spectroscopy (EIS) data, collected at − 1.0 and − 1.3 V vs. SCE (Fig. 3B & C), highlight the presence of at least three time constants. The high-frequency contribution is plausibly due to geometric characteristics of the film (Rp and Cp), while the others can be related to the electrochemical discharge of protons (Rct and Cdl) and to the involvement and adsorption of intermediates on the electrode surface (Rϕ and Cϕ), especially in light of the high Tafel slope measured [11,15]. A tentative fitting was carried out by using a complex equivalent circuit that comprises three RC meshes, as reported in Fig. 3C. Results of fitting, for two different potentials, are summarized in Fig. 3D. By moving towards more cathodic potential values, the fraction of surface catalytically active towards the hydrogen evolution decreases, as indicated by a decrease of Rp and Cp (both related to pores) and Cdl: this result may be due to the formation of gas within the pores, with obvious clogging of a fraction of the active surface. Looking at trends of Rϕ and Cϕ, the role of intermediates is evident: it becomes more important when lowering the potential from −1.0 to −1.3 V vs. SCE, possibly because of the characteristics of electroactive sites (which seem to stabilize the intermediate). SEM-EDX and RBS analyses were repeated after the HER investigation, with the aim of checking the morphology and chemical composition of the electrode (insight of Fig. 3A): the surface appears micro-
12
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
Fig. 3. (A) Polarization curve for RuN in 1 M NaOH solution enriched with hydrogen through a pre-polarization of 30 min at 1000 A m−2; inset: SEM image of the surface after the electrochemical treatment. (B, C) Nyquist and Bode plots collected at −1.0 and −1.3 V vs. SCE, respectively, under the same experimental conditions of (A). (D) Results of EIS fitting: system modeled considering the equivalent circuit reported in the inset of (C).
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
fractured, after the intense HER, and the pyramidal units that characterized the pristine surface look sintered. Analogous modifications were previously noted also on sputtered oxide layers synthesized at room temperature [16]. Concerning the chemical composition of the coating, no variation could be evidenced by EDX analyses, carried out on surface areas of approximately 1.44 mm2 in two different regions (one exposed to the electrochemical treatment, while the other had been protected with a ribbon of Teflon). 4. Conclusions RuN electrodes appear rather stable against dissolution, independently of pH, but show to possess a great stability only in alkaline environment; in acidic media, an oxide forms on their surface when polarized at values higher than about 0.2 V vs. SCE. Films are stable under hydrogen evolution conditions, and present an interesting catalytic activity, comparable with those of the most famous and active catalysts. The mechanistic pathway is likely to involve a significant coverage by adsorbed reaction intermediates. The impedance analysis highlighted at least three time constants: the high-frequency one is reasonably due to porosity and geometric features of the surface, while the contribution at mid-range frequencies is probably due to the adsorption of hydrogen radicals. The higher stability shown by films synthesized through rfmagnetron sputtering, in comparison with other synthetic procedures, may allow further investigations, possibly extended to mixtures, with potential applications in energetics and sensoring. Conflict of interest There is no conflict of interest. Acknowledgments The authors would like to thank Prof. Achille De Battisti (University of Ferrara) for the important and fruitful discussion of results reported in this manuscript.
13
References [1] B. Avasarala, P. Haldar, Electrochemical oxidation behavior of titanium nitride based electrocatalysts under PEM fuel cell conditions, Electrochim. Acta 55 (2010) 9024–9034. [2] Y. Xie, Y. Wang, H. Du, Electrochemical capacitance performance of titanium nitride nanoarray, Mater. Sci. Eng. B 178 (2013) 1443–1451. [3] X. Chen, X.P. Gao, H. Zhang, Z. Zhou, W.K. Hu, G.L. Pan, H.Y. Zhu, T.Y. Yan, D.Y. Song, Preparation and electrochemical hydrogen storage of boron nitride nanotubes, J. Phys. Chem. B 109 (2005) 11525–11529. [4] A.L. Ivanovskii, Platinum group metal nitrides and carbides: synthesis, properties and simulation, Russ. Chem. Rev. 78 (2009) 303–318. [5] L. Hultman, Thermal stability of nitride thin films, Vacuum 57 (2000) 1–30. [6] M.G. Moreno Armenta, J. Diaz, A. Martinez-Ruiz, G. Soto, Synthesis of cubic ruthenium nitride by reactive pulsed laser ablation, J. Phys. Chem. Solids 68 (2007) 1989–1994. [7] S. Bouhtiyya, R. Lucio Porto, B. Laik, P. Boulet, F. Capon, J.P. Pereira-Ramos, T. Brousse, J.F. Pierson, Application of sputtered ruthenium nitride thin films as electrode material for energy-storage devices, Scripta Mater. 68 (2013) 659–662. [8] E. Cattaruzza, G. Battaglin, P. Riello, D. Cristofori, M. Tamisari, On the synthesis of a compound with positive enthalpy of formation: zinc-blende-like RuN thin films obtained by rf-magnetron sputtering, Appl. Surf. Sci. (2014), http://dx.doi.org/10. 1016/j.apsusc.2014.09.158. [9] P. Barricelli, M., Boccardo, G. Maranelli, Italian Patent Appl. n. PD 2008A000104 (2008). [10] L.R. Doolittle, Algorithms for the rapid simulation of Rutherford backscattering spectra, Nucl. Instrum. Meth. B 9 (1985) 344–351. [11] D. Rosestolato, J. Fregoni, S. Ferro, A. De Battisti, Influence of the nature of the electrode material and process variables on the kinetics of the chlorine evolution reaction. The case of IrO2-based electrocatalysts, Electrochim. Acta 139 (2014) 180–189. [12] A. Lasia, Hydrogen evolution reaction, Electrocatalysis, the hydrogen oxidation/ evolution reaction, Handbook of Fuel Cells, John Wiley and Sons Ltd., 2010. [13] J.K. Norskov, T. Bligaard, A. Logadottir, J.R. Kitchin, J.G. Chen, S. Pandelov, U. Stimming, Trends in the exchange current for hydrogen evolution, J. Electrochem. Soc. 152 (2005) J23–J26. [14] D. Tang, J. Lu, L. Zhuang, P. Liu, Calculations of the exchange current density for hydrogen electrode reactions: a short review and a new equation, J. Electroanal. Chem. 644 (2010) 144–149. [15] B.V. Tilak, B.E. Conway, Analytical relations between reaction order and Tafel slope derivatives for electrocatalytic reactions involving chemisorbed intermediates, Electrochim. Acta 37 (1992) 51–63. [16] G. Battaglin, D. Rosestolato, S. Ferro, A. De Battisti, Characterization of IrO2 –SnO2 films prepared by physical vapor deposition at ambient temperature, Electrocatalysis 4 (2013) 358–366.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrochemical properties of stoichiometric RuN film prepared by rf-magnetron sputtering: A preliminary study Davide Rosestolato a,⁎, Giancarlo Battaglin b, Sergio Ferro a a b
Dipartimento di Scienze Chimiche e Farmaceutiche, Università degli Studi di Ferrara, via Fossato di Mortara 17, 44121 Ferrara, Italy Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca' Foscari Venezia, Via Torino 155/B, 30170 Mestre— VE, Italy
a r t i c l e
i n f o
Article history: Received 2 September 2014 Received in revised form 18 September 2014 Accepted 29 September 2014 Available online 5 October 2014 Keywords: Ruthenium mononitride Rf-magnetron sputtering Hydrogen evolution reaction Thin film stability Adsorbed intermediates
a b s t r a c t The electrochemical properties and stability of ruthenium mononitride (RuN) thin films were studied. Coatings of RuN were synthesized on electropolished titanium supports by rf-magnetron sputtering. RuN electrodes appear rather stable against dissolution, independently of pH, but show to possess the greatest stability only in alkaline environment. Under hydrogen evolution conditions the films show relevant catalytic properties, comparable with Pt, Pd and Ru/Ir derivatives; a significant coverage by adsorbed reaction intermediates is involved. These electrodes are of potential application in energetics and sensoring. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The persistent and growing interest for materials, to be used in catalysis and energy storage applications, has prompted the research towards the study of platinum group metal nitrides. Their structural and morphological properties, combined with electronic features [1–3], make them attractive for different purposes. The features of nitrides and carbides of noble metals have been reviewed by Ivanovskii [4] in 2009. Since these compounds are generally characterized by a positive enthalpy of formation, accompanied by sensitivity to high temperatures [5], the reactive sputtering synthesis appears suitable for obtaining films with a well-defined stoichiometry. In this context, the present study has been focused on ruthenium nitride. Previous works have shown that stoichiometric RuN can be synthesized by pulsed laser ablation [6] and dc-magnetron sputtering [7]; the possibility to exploit also the rf-magnetron sputtering has been highlighted only recently [8]. In this paper, an electrochemical characterization of RuN films was carried out by means of cyclic voltammetry, polarization curves and impedance spectroscopy, under hydrogen evolution conditions, with the aim of obtaining information regarding the stability and catalytic activity of this material, which could be used as a cathode in different applications.
RuN films were synthesized by radio-frequency reactive sputtering, at room temperature, using the optimized procedure described in Ref. [8]. Samples were deposited both on electropolished (following the procedure described in Ref. [9]) titanium supports (2 × 2 cm), as well as onto silica sheets (1 × 1 cm). The thickness of obtained films was measured with a Tencor alpha-step 500 profilometer, on a step obtained by covering a small portion of the substrate during film deposition. The film composition was analyzed by Rutherford Backscattering Spectroscopy (RBS) at Laboratori Nazionali INFN-Legnaro (Padova, Italy) using a 2 MeV 4He+ beam with a spot size of about 2 mm2. Depth profiles were modeled by means of the RUMP computer code [10]. X-ray diffraction (XRD) experiments were accomplished with a Bruker D8 equipped with a Cu Kα source; a step size of 0.02° was adopted. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX) characterizations were performed with a FEIQUANTA 400 at the electron microscopy center of the University of Ferrara (Ferrara, Italy). In order to carry out the electrochemical investigation, a triple-body cell has been adopted, whose geometry has been described e.g. in Ref. [11]. All chemicals used were of high purity degree.
3. Results and discussion
⁎ Corresponding author. Tel.: +39 0532 455166. E-mail address: [email protected] (D. Rosestolato).
http://dx.doi.org/10.1016/j.elecom.2014.09.019 1388-2481/© 2014 Elsevier B.V. All rights reserved.
The analysis of the RBS spectrum shown in Fig. 1A confirms that prepared coatings possess a Ru:N stoichiometry very close to 1:1, while measurements with the profilometer evidenced a thickness of
10
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
Fig. 1. (A) RBS, (B) XRD and (C) SEM analyses for RuN sample synthesized on titanium; in (A), experimental points are shown as dots, while the continuous line is the result of RUMP fitting.
0.55 μm. Based on signal profiles, for which no concentration gradient can be recognized, a quite homogeneous film growth is evident. In agreement with previous literature data [6–8], the XRD investigation (Fig. 1B) indicated that a cubic crystalline phase has been obtained. The so-called face-centered cubic (fcc) packing is compatible with a number of structures: those of sodium chloride, anti-fluorite and zincblende are generally taken into account [6]. Of course, the charge density characteristic of considered species justifies different volumes for the lattice cells. In Fig. 1B, an important asymmetric peak is evident at 2ϑ ≈ 34°, which is plausibly due to the convolution of signals from Ti (support) and from the {111} planes of RuN cubic lattice. A number of less intense signals can be further recognized: peaks corresponding to the {200} and {220} reflective planes appear at 2ϑ ≈ 40° and 58°, respectively; eventually, the {311} family plane appear at about 70°. Focusing on the peak associated to the {220} family plane, which falls at about 57.93° in a region free from other signals, the calculation of the a = b = c cell parameter gave a value of 0.450 nm, typical of the zinc-blende structure. This outcome is in good agreement with evidences obtained in a previous work [8]. Moreover, based on the Scherrer equation:
t¼
λ FWHM cos ϑ
the average crystallite size can be estimated to be about 18 nm. SEM analyses (Fig. 1C) show a rough surface, well organized in pseudo-pyramidal structures having a triangular base. A sub-micro or even nano-porosity is evidently present along the whole material surface, with possible effects on its electrochemical properties.
As usual, the electrochemical investigation was started by collecting some preliminary CV curves; samples were preliminarily conditioned, by immersion in 1 M NaClO4 solution for 24 h. Since it has been reported that RuN is not stable in alkaline and acidic media [6], a neutral environment has been initially considered, limiting the window of polarization to the interval of water stability. As shown in Fig. 2A (CV in 1 M NaClO4), the electrode material seems to gradually change, reaching a stabilization after 400 scans. Its resistivity seems to decrease, and the shape of the CV curve becomes progressively similar to that of RuO2. The õelectrode was then investigated in 1 M HClO4 solution: after 400 scans, the appearance of broad signals characteristic of Ru(II)–Ru(III) and Ru(III)–Ru(IV) oxidation state transitions is evident (Fig. 2B). However, returning to the neutral environment, the response of the electrode turned out to be the same as previously obtained after the first 400 cycles. EDX analyses, performed on the “as deposited” and on the “electrochemical investigated” parts of the same sample, qualitatively indicate for the latter an increase in the oxygen content at the expense of the nitrogen one, thus confirming the hypothesis put forward on the basis of the voltammetric data. Since the electrode oxidation occurred without morphological modifications (the SEM analysis did not reveal any difference) the persistence of the characteristic columnar pyramidalterminated structure can be taken as an indication that oxidation occurred only in a thin, near surface region, of the RuN zinc-blend structure. After etching the first layers of the surface with an Ar beam, a composition close to that of the as prepared film was obtained, thus proving that the oxidation involves only the most external surface. Heavier changes (towards the more stable thermodynamic form) can probably be obtained by extending the electrochemical polarization for longer times.
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
11
Fig. 2. CV curves in different electrolytes and at different scan rates: (A) in 1 M NaClO4 at 100 mV s−1; (B) 400 cycles in 1 M solutions at 100 mV s−1; (C) in 1 M NaOH at 50 mV s−1. Indications in chart legends are reported in chronological order.
A new sample, prepared during the same round of deposition, was then studied, limiting the window of applied potential (from − 0.2 to 0.2 V vs. SCE) in order to avoid the modifications that seem to lead to the formation of the oxide. In this second turn of experiments, the scan rate was also reduced to 50 mV s−1, in order not to introduce kinetic constrictions that could affect the possible modification of the electrode surface. As shown in Fig. 2C, the material response reached a good stability after 400 cycles. Two aspects deserve to be underlined: the sample appears stable to dissolution, over the whole pH interval, and the alkaline environment seems to favor the stabilization of the external surface of the nitride. A confirmation was obtained by both EDX and RBS, which proved that the stoichiometry of the nitride surface did not change; as a result, the previous conditions (applied potential, and acidic pH) were responsible for the oxidation. Moreover, samples were left in the solutions for several hours, without showing any significant modification in composition, structure and thickness: this allows excluding any sort of corrosion/dissolution or modification of material, due to the chemical action of electrolytes. As a further way to characterize the material, the hydrogen evolution reaction (HER) was investigated: polarization curves were collected under steady-state regime in 1 M NaOH; in order to obtain reproducible data, the solution nearby the electrode surface was preliminarily enriched in dissolved hydrogen, by polarizing the electrode at a current density of 1000 A m−2 for 30 min. The steady-state polarization was performed step by step, moving from −1.24 V to −1.07 V vs. SCE with steps of 5 mV, and maintaining each polarization for 5 min before registering the current value. An example of obtained Tafel plots, corrected for the ohmic drop of the solution, is shown in Fig. 3A. A rather good linearity has been detected, which extended for a decade of current; however, the quite high value of Tafel slope is in disagreement with the conventional Volmer–Tafel and Volmer–Heyrovsky mechanisms, commonly found on other electrode materials [12]. The
exchange current density, estimated by extrapolating the high-field approximation line, resulted to be around 0.14 mA cm−2; in order to normalize the latter to the real surface, SEM images shown in Ref. [8] were considered, assuming that the film is formed by columnar pyramidal structures. Based on an approximated roughness factor of 1.15, an exchange current density of ≈0.12 mA cm−2real is obtained, which is comparable to values collected in acidic media at Pt, Pd, Rh and Ir [13], as well as in alkaline media at Pt electrodes (Ref. [14] and references therein). Electrochemical Impedance Spectroscopy (EIS) data, collected at − 1.0 and − 1.3 V vs. SCE (Fig. 3B & C), highlight the presence of at least three time constants. The high-frequency contribution is plausibly due to geometric characteristics of the film (Rp and Cp), while the others can be related to the electrochemical discharge of protons (Rct and Cdl) and to the involvement and adsorption of intermediates on the electrode surface (Rϕ and Cϕ), especially in light of the high Tafel slope measured [11,15]. A tentative fitting was carried out by using a complex equivalent circuit that comprises three RC meshes, as reported in Fig. 3C. Results of fitting, for two different potentials, are summarized in Fig. 3D. By moving towards more cathodic potential values, the fraction of surface catalytically active towards the hydrogen evolution decreases, as indicated by a decrease of Rp and Cp (both related to pores) and Cdl: this result may be due to the formation of gas within the pores, with obvious clogging of a fraction of the active surface. Looking at trends of Rϕ and Cϕ, the role of intermediates is evident: it becomes more important when lowering the potential from −1.0 to −1.3 V vs. SCE, possibly because of the characteristics of electroactive sites (which seem to stabilize the intermediate). SEM-EDX and RBS analyses were repeated after the HER investigation, with the aim of checking the morphology and chemical composition of the electrode (insight of Fig. 3A): the surface appears micro-
12
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
Fig. 3. (A) Polarization curve for RuN in 1 M NaOH solution enriched with hydrogen through a pre-polarization of 30 min at 1000 A m−2; inset: SEM image of the surface after the electrochemical treatment. (B, C) Nyquist and Bode plots collected at −1.0 and −1.3 V vs. SCE, respectively, under the same experimental conditions of (A). (D) Results of EIS fitting: system modeled considering the equivalent circuit reported in the inset of (C).
D. Rosestolato et al. / Electrochemistry Communications 49 (2014) 9–13
fractured, after the intense HER, and the pyramidal units that characterized the pristine surface look sintered. Analogous modifications were previously noted also on sputtered oxide layers synthesized at room temperature [16]. Concerning the chemical composition of the coating, no variation could be evidenced by EDX analyses, carried out on surface areas of approximately 1.44 mm2 in two different regions (one exposed to the electrochemical treatment, while the other had been protected with a ribbon of Teflon). 4. Conclusions RuN electrodes appear rather stable against dissolution, independently of pH, but show to possess a great stability only in alkaline environment; in acidic media, an oxide forms on their surface when polarized at values higher than about 0.2 V vs. SCE. Films are stable under hydrogen evolution conditions, and present an interesting catalytic activity, comparable with those of the most famous and active catalysts. The mechanistic pathway is likely to involve a significant coverage by adsorbed reaction intermediates. The impedance analysis highlighted at least three time constants: the high-frequency one is reasonably due to porosity and geometric features of the surface, while the contribution at mid-range frequencies is probably due to the adsorption of hydrogen radicals. The higher stability shown by films synthesized through rfmagnetron sputtering, in comparison with other synthetic procedures, may allow further investigations, possibly extended to mixtures, with potential applications in energetics and sensoring. Conflict of interest There is no conflict of interest. Acknowledgments The authors would like to thank Prof. Achille De Battisti (University of Ferrara) for the important and fruitful discussion of results reported in this manuscript.
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