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Influence of the implantation of transition metal ions on the electrocatalytic activity of carbon materials
534
Nuclear Instruments and Methods in Physics Research B28 (1987) 534-539 Norm-Holland. Amsterdam
INFLUENCE OF THE IMPLANTATION OF TRANSITION METAL IONS ON THE ELECTROCATALYTIC ACTIVITY OF CARBON MATERIALS
V.V. POPLAVSKIJ Byelorussian Technological Institute, Minsk, 220630 USSR Received 9 September 1986 and in revised form 9 June 1987
The influence of implantation of titanium, vanadinrn chromium iron, cobalt, nickel, zirconium, molybdenum, silver, tungsten, iridium and platinum ions on the catalytic activity of poly~~s~l~ne graphite and glassy carbon for the electr~he~c~ hydrogen evolution reaction has been studied. The factors causing an increase of material activity as a result of ion beam modification were determined. Implanted layers were investigated by XPS and voltammetric measwements. The formation of metastable states of implanted transition metal atoms in the carbon matrix has been established. These states are characterized by an unexpected high extent of unoccupancy of d-electronic valence levels. The transition of the implanted system in the equilibrium state occurs by metal atom diffusion and precipitation. Active sites of electronic exchange in the near-surface substrate region are bound to implanted metal atoms or their precipitates. These phenomena result in an increase of adsorptivity and considerable decrease of hydrogen overpotential on modified electrodes.
1. Introduction Carbon materials are promising as electrode elements in electrolyzers and fuel cells due to their high chemical stability, perfect electric conductivity and comparatively low cost. In this case the electrode activity with respect to hydrogen evolution is increased by means of metal-containing coatings decreasing the hydrogen overpotential. In this connection, it is reasonable to modify the surface of carbon materials by implantation of el~tr~at~ytic d-transition metal ions. As compared to the conventional surface modification methods, the application of ion implantation offers the following advantages: (1) the possibility of obtaining me&stable surface phases possessing apron properties; (2) the formation of active electron-exchange sites associated with implanted atoms; (3) a small amount of implanted activating impurity; (4) workability, since, as a rule, the surface mo~fication is performed by one technological procedure. This paper deals with the implantation effects of titanium, vanadium, iron, cobalt, nickel, chromium, zirconium, molybdenum, silver, tungsten, iridium and platinum ions on the catalytic activity of polycrystalline graphite and glassy carbon with respect to the reaction of electrochemical hydrogen evolution. The interaction between accelerated ions of transition metals and a carbon matrix is both of practical and scientific interest. The system under investigation presents a unique model to study the effects of a nonequilibrium process such as 0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
ion implantation since under equilibrium conditions the d-transition metals are absolutely insoluble in carbon and at low metal concentrations do not react with carbon in the solid phase [1,2]. It is to be noted that the attempts made earlier to modify the electrocatalytic properties of carbon materials B-91 by use of ion ~plantation (without ion mixing) most commonly did not result in a considerable change in activity. The mechanism of the ion implantation effect on the catalytic properties of solid surfaces has not been explained so far. The researchers have established only the predominant role of specific process effects stimulated by the nature of the ion [lo-131.
2. Experimental The ion implantation was carried out at an accelerating voltage of 10 kV in the range of 6 X lOI2 to 1.7 X 1Or6 ions cme2. The content of implanted metals in the surface area was calculated using Rutherford backscattering spectroscopic data on He” ions. Peak metal concentrations were not higher than 10 at.%. A study of the electrocatalytic activity of modified materials was carried out for the electrochemical hydrogen evolution from 1M H,S04 at room temperature. The solution was prepared by using reagent grade chemicals and redistilled water. The hydrogen evolution curves obtained by cathode polarization were taken under potentiokinetic conditions at a potential sweep
535
V, V. Poplavskij / Electrocatalytic activity of transition-metal implanted carbon rate of 0.4 mV/s. A saturated silver chloride electrode (SSCE) is used as a reference electrode. The preliminary cleaning of the electrodes studied was done by means of holding them for 30 min at a cathode potential of - 0.4 V. The solution was neither stirred nor de-aerated. It was taken into account that the maximum diffusion current density for oxygen reduction under the experimental conditions had been determined both through calculation [14] and experimentally [8] to account for only 5 0.1% of the cell current density and to be negligible with respect to the hydrogen evolution involved. The dependence of electrode electrochemical behavior upon implanted metal concentration on a surface area which is changing due to corrosion during voltammetric measurements has been studied. An example of such an investigation is given in sect. 3.1 where surface adsorption properties of carbon materials alloyed with silver ions are discussed. However, the dependence only holds qualitatively, for the electrode processes concerned have been seen to penetrate to a considerable depth in the case of compact carbon materials (see ref. [15]), with a substantial amount of modified surface area working. The electron structure features of implanted atoms were studied by two independent methods: voltammetry in 1M H,SO, and X-ray photoelectron spectroscopy (ES2401, USSR). The emission of photoelectrons was excited by Mg K, radiation under a pressure of lop7 Pa. Photoelectronic spectra where obtained for samples held for several months after having been implanted at room temperature both without and with preliminary cleaning by means of etching for 30 min with Ar+ ions of 800 eV energy, the ionic current density being 80 PA cmW2.
3. Results
? .l-
0 L
vs.
.8 SSCf
v
Fig. 1. Cyclic voltammograms for electrodes of unimplanted glassy carbon (A), glassy carbon implanted with argon ions, 70 keV, 5.6 x lOI cmm2 (B), and silver ions, 1.4X 1015 cm-* (C) in 1M H,SO,. The potential sweep rate is 40 mV/s.
and discussion
3. I. Electrochemical
properties
A high adsorptivity relative to hydrogen has been found to be a distinctive feature of an ion-alloyed surface. It was illustrated that cyclic voltammograms for carbon materials alloyed with silver ions have current peaks specified by hydrogen adsorption and desorption (figs. 1 and 2) although the silver itself does not chemisorb the hydrogen [16]. The peaks appear after the hydrogen evolution polarization curves have been recorded. The peak intensity increases during the cycling, with electrode adsorption properties being activated due to greater active surface area involvement. During the cycling, done with successively greater anode potentials, the ionization current peak intensity for desorbing hydrogen first increases and later, as the electrode surface dissolves, decreases. As a result of complete depletion of
Fig. 2. Cyclic voltammograms for electrodes of unimplanted graphite (A), and graphite implanted with silver ions, 1.5 x 1015 cmm2 (B) in 1M H,SO,. The potential sweep rate is 40 mV/s.
536
V. V. Poplavskij / Electrocatalytic activity of transition-metal implanted carbon
the modified layer cyclic voltammograms are like those for unmodified carbon materials having no peaks due to adsorbed hydrogen. This behavior corresponds to the implanted metal distribution profile. Due to a high adsorptivity, the discharge process of hydrogen ions on the electrode surface is facilitated, resulting in a decrease of the hydrogen overpotential. This an be visualized by cathodic polarization curves (figs. 3 and 4) taken in the region of high current densities j, which are of interest for commercial electrolysis. The electrocatalytic surface activity for j 2 3 kA/m2 substantially exceeds that of an unmodified carbon surface. As the current density increases the adsorbed hydrogen gradually accumulates on the material surface, which hinders the movement of hydrogen ions from the solution to the cathode, causing an increase in overpotential. The two effects are especially strong on glassy carbon alloyed with iron ions (fig. 3f), the latter being strong also in the case of glassy carbon alloyed with titanium ions (fig. 3g). The adsorptivity increases with an increasing implanted metal content. Keeping the samples for a long time at room temperature has been found to lead to a progressive change in
3
2
r
i -bq
0 1
.5 -E,s
RHE/V
Fig. 4. Cathodic polarization curves of the hydrogen evolution from 1M H,SO, at 25°C on graphite electrodes. Use was made of electrodes implanted with ions of (A) platinum, 1.5 X 1015 cm-2; (B) vanadium, 1.6 X 10” cmW2; (C) iridium, 8.5 X 1014 cmm2; (D) unimplanted graphite. The curves were taken (a) immediately after ion implantation and (b) after ageing at room temperature. The potential sweep rate is 0.4 mV/s.
their properties. The surface activity of the materials under investigation after recovery correlates with that of transition metals and in most cases remains high as well. In this case it increases considerably for the region of high current densities in the cell (see fig. 4). 3.2. Electron structure of implanted atoms and its features
.s
--E”S
1.8
R”:,”
Fig. 3. Cathodic polarization curves of the hydrogen evolution from 1M H,SO, at 25OC on glassy carbon electrodes. Use was made of electrodes implanted with ions of (A) silver, 1.8 X lOI cm- ‘; (B) vanadium, 1.2 x 1015 cm -2; (C) platinum, 1.3 ~10’~ cmM2; (E) nickel, 2.1 X lOI cm- 2; (D) tungsten, 1.1 x 1016 cm-‘; (F) iron, 7.9 X 10” cmm2; and (G) titanium, 1.6 x 1016 cmm2; (H) unimplanted glassy carbon. The potential sweep rate is 0.4 mV/s.
The electron structure studies of sample surfaces showed that the implanted atoms of transition metals are in metastable states exhibiting an unexpected high extent of unoccupancy of valence electron levels. In the photoelectron spectra excited from the core levels of implanted metal atoms, along with spectral lines corresponding to the metal state, there are lines of comparable intensity with a higher binding energy, indicating that part of the atoms have unoccupied valence electron levels (figs. 5-8). The energy shift, AE, is significant. For level Fe 2p, its value amounts, e.g., to 4.8 eV; for Ni 2p, it is 5.6 eV; for Pt 4d, 4.0 eV. The experiments on anodic oxidation of ion-alloyed glassy carbon surfaces revealed a considerable increase (- 1 v) in the oxidation potential of implanted metals (figs. 9 and lo), which is characteristic for the oxidation of implanted atoms in the form of high-valence ions.
V. V. Poplavskij I
I
/ Electroeatalytic
activity of transition-metal
531
implanted carbon
I
I
AE Fe 2~72
fi
Fe 2p 3/2 I
I
700 hdrng
I
720
I
I 740
energy/ev
Fig. 5. Spectrum of photoelectrons excited from the 2p level of iron atoms implanted into glassy carbon. The ion dose is 9.0X 1015 cmm2. ES2401 spectrometer; X-rays, Mg K,, 1253.6 eV.
The analysis of the results obtained and their comparison with literature data permit to conclude that the effective valency of implanted transition metal atoms into a carbon matrix is higher than in the case of carbide and other known systems that incorporate a transition metal and carbon. The chemical shift, for example, for transition metal core levels in carbides is as low as 2 eV [17,18]; the Pt 4f level shift in the PtC, phase obtained by means of platinum sputtering in a carbonaceous atmosphere is s 0.16 eV [19]. The energy shift for core levels of metals deposited in submonolayer amounts at the surface of inert materials, including carbon ones, is as low as ca. 1.5 eV, with the shift amount being dependent on the extent of the coverage of the deposited metal and cluster size [20-241 (see also fig. 11). In the system studied, two spectral lines of the same energy level of implanted metal atoms have been recorded due to their equilibrium and metastable states respectively, with the energy shift being dependent on the nature of the metal.
850
870
890
Binding energy/eV Fig. 6. Spectrum of photoelectrons excited from the 2p level of nickel atoms implanted into glassy carbon. The ion dose is 1.6 X 1016 cme2. ES2401 spectrometer; X-rays, Mg K,, 1253.6 eV.
I 310
Binding
I
I
I
330
energy/Y
Fig. 7. Spectra of photoelectrons excited from the 4d level of platinum atoms implanted into glassy carbon: (A) after implantation and keeping at room temperature; (B) after annealing of the studied sample at 300 o C within 2 h. The ion dose is 1.5 X 1016 cm-‘. ES2401 spectrometer; X-rays, Mg K,, 1253.6 eV.
The implanted system gradually changes to an equilibrium state as a result of diffusion of metal atoms and precipitation as a consequence of recovery at room temperature. The precipitation can be activated by thermal annealing. For instance, annealing at 300 o C within 2 h considerably decreases the intensity of spectral lines corresponding to the platinum atoms implanted into glassy carbon with highly unoccupied electron states (fig. 7). An analysis of the electronic configuration of carbon and oxygen atoms as well as of components of the modified surface has revealed the formation of metastable states of implanted metallic atoms due to local electronic interaction with the carbon matrix. Thus, implantation results in the formation of metastable states of transition metal atoms implanted into the carbon matrix that are characterized by unexpected unoccupied valence electron levels. Upon implantation of insoluble impurity atoms, strong crystalline local fields are developed which cause not only variations in the configuration of electron levels of transition metal atoms observed earlier [25,26] but also a delocalization of their valence electrons. The implanted metal atoms exhibiting highly unoccupied d-electron levels are associated with coordinately unsaturated sites of an electron exchange in the material surface layer. This is the reason for an increase in adsorptivity and the characteristics observed in electrocatalytic hydrogen evolution. Upon the system’s transition to an equilibrium state the active chemisorption sites appear to be associated with metal precipitates and the surface properties of carbon materials correlate with the properties of modifying metals. The bonding strength of the adsorbed hydrogen
mea Bi
295.118
ndi ng Energy
3% UB
315.86
CeV)
Fig- 8. Spectrum of photoelectrons excited from the Is levef of carbon atoms and the 4d level of iridium atoms in the mrface area of &SSy carbon ~p~~n~~ with iridium ions, The ioix dose is 1.7 X lox6 cm --2. VG ESCAIAE-5 spectrometer; X-rays, Al K,, 14X6.6 eV.
t
Fig. 9. PotentM variation in the armdic oxidation of iron atoms implanted into glassy carbon. The ion dose is 7.9 X lOI cm-‘. 1M F$,SO,. The potential sweep rate is 40 mV/s.
’
I
I
I
I 1
Fig. 10. Potential variation in the anodic oxidatiozx of nickel atoms implanted into glassy carbon. The ion dose is 2.1 X 1Oi6 cm-*. 1M H,SO,. The potential sweep rate is 40 mV/s.
V. V. Poplavskij
/ Electrocatalytic
activity of transition-metal
implanted carbon
[31 M. Voinov, D. Bier
310
Binding
330
energy
/ eV
Fig. 11. Spectrum of photoelectrons excited from the 4d level of platinum atoms deposited on a glassy carbon surface. 2 X lOi -2; 0.05M H,PtCl, .6H,O + 0.02% Pb(C,H,O,), ;:(OH) s solution. ES2401 spectrometer; X-rays, Mg K, 1253.6 eV.
at the modified surface being different, hydrogen evolution reaction mechanisms also differ. Despite the difference in the nature of cathode processes, a high catalytic activity of the modified electrodes is retained in both cases. The author would like to thank A.A. Ivko and P.D. Zhuk for assistance in performing the X-ray photoelectron spectroscopy experiments.
References [l] 1.1. KorniIov, N.M. Matveeva, L.I. Pryakhina and R.S. Polyakova, MetaIlochemicaI Properties of the Periodic System Elements (Nauka, Moscow, 1966). [2] T.Y. Kosolapova, Carbides: Properties, Production, and Applications, transl., N.B. Yaughn (Plenum, New York, 1971).
539
and H. Tannenberger, Proc. Symp. on Electrocatalysis (Electrochem. Sot., New York, 1974) p. 268. [41 G.K. Wolf, Chem. Ing. Techn. 54 (1982) 23. 151 G.K. Wolf, K. Zucholl, H. Folger and W.E. O’Grady, Nucl. Instr. and Meth. 209/210 (1983) 835. [61 G.K. Wolf, K. Zucholl and H. Folger, Nucl. Instr. and Meth. Bl (1984) 240. 171 K. ZucholI, G.K. Wolf and H. Schmiedel, Proc. Int. Conf. on Surface Modification by Metal Ion Beams, Heidelberg (1984) p. 327. PI K. Takahachi, K. Yoshida and M. Iwaki, Nucl. Instr. and Meth. B7/8 (1985) 526. PI K. Takahachi, K. Yoshida and M. Iwaki, J. Chem. Sot. Jap., Chem. and Ind. Chem. no. 6 (1985) 1055. WI W.A. Grant, Inst. Phys. Conf. Ser. no. 28 (1976) 127. Ml G.K. Wolf, Top. Curr. Chem. 85 (1979) 1. WI G.K. Wolf, Radiat. Eff. 48 (1980) 237. [I31 G.K. Wolf, Nucl. Instr. and Meth. 182/183 (1981) 875. D41 N.P. Zhuk, A Theoretical Course on Metal Corrosion and Protection (Metallurgia, Moscow, 1976). [I51 Y.B. Vasihyev, L.S. Kanevskij, V.I. Lushnikov and A.M. Skundin, Electrochimia (USSR) 13 (1977) 440. WI S.R. Morrison, The Chemical Physics of Surfaces (Plenum, New York and London, 1977). P71 V.V. Nemoshkalenko and V.G. AIyoshin, Electronic Spcctroscopy of Crystals (Naukova Dumka, Kiev, 1976). WI L. Leclercq, Proc. NATO Conf. on The Advances in Surface Properties and Catalysis by Non-Metals, LiUe (1983) p. 433. WI A. Hecq, T. Robert, M. Hecq, J.P. Delrue, J.J. Pireaux and R. Caudano, J. Less-Common Metals 80 (1981) 83. WI A. Fritsch and P. LegarC, Surf. Sci. 162 (1985) 742. [21] F. Parmigiani, E. Kay, P.S. Bagus and C.J. Nelin, J. Electron Spectrosc. Relat. Phenom. 36 (1985) 257. [22] T.T.P. Cheung, Surf. Sci. 140 (1984) 151. [23] T.T.P. Cheung, Chem. Phys. Lett. 110 (1984) 219. [24] G.K. Wertheim, S.B. DiCenzo, D.N.E. Buchanan and P.A. Bemett, Sol. Stat. Commun. 53 (1985) 377. [25] K.S. Kim and N. Winograd, Chem. Phys. Lett. 30 (1975) 91. [26] R.A. Gibbs, N. Winograd and V.Y. Young, J. Chem. Phys. 72 (1980) 4799.
Nuclear Instruments and Methods in Physics Research B28 (1987) 534-539 Norm-Holland. Amsterdam
INFLUENCE OF THE IMPLANTATION OF TRANSITION METAL IONS ON THE ELECTROCATALYTIC ACTIVITY OF CARBON MATERIALS
V.V. POPLAVSKIJ Byelorussian Technological Institute, Minsk, 220630 USSR Received 9 September 1986 and in revised form 9 June 1987
The influence of implantation of titanium, vanadinrn chromium iron, cobalt, nickel, zirconium, molybdenum, silver, tungsten, iridium and platinum ions on the catalytic activity of poly~~s~l~ne graphite and glassy carbon for the electr~he~c~ hydrogen evolution reaction has been studied. The factors causing an increase of material activity as a result of ion beam modification were determined. Implanted layers were investigated by XPS and voltammetric measwements. The formation of metastable states of implanted transition metal atoms in the carbon matrix has been established. These states are characterized by an unexpected high extent of unoccupancy of d-electronic valence levels. The transition of the implanted system in the equilibrium state occurs by metal atom diffusion and precipitation. Active sites of electronic exchange in the near-surface substrate region are bound to implanted metal atoms or their precipitates. These phenomena result in an increase of adsorptivity and considerable decrease of hydrogen overpotential on modified electrodes.
1. Introduction Carbon materials are promising as electrode elements in electrolyzers and fuel cells due to their high chemical stability, perfect electric conductivity and comparatively low cost. In this case the electrode activity with respect to hydrogen evolution is increased by means of metal-containing coatings decreasing the hydrogen overpotential. In this connection, it is reasonable to modify the surface of carbon materials by implantation of el~tr~at~ytic d-transition metal ions. As compared to the conventional surface modification methods, the application of ion implantation offers the following advantages: (1) the possibility of obtaining me&stable surface phases possessing apron properties; (2) the formation of active electron-exchange sites associated with implanted atoms; (3) a small amount of implanted activating impurity; (4) workability, since, as a rule, the surface mo~fication is performed by one technological procedure. This paper deals with the implantation effects of titanium, vanadium, iron, cobalt, nickel, chromium, zirconium, molybdenum, silver, tungsten, iridium and platinum ions on the catalytic activity of polycrystalline graphite and glassy carbon with respect to the reaction of electrochemical hydrogen evolution. The interaction between accelerated ions of transition metals and a carbon matrix is both of practical and scientific interest. The system under investigation presents a unique model to study the effects of a nonequilibrium process such as 0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
ion implantation since under equilibrium conditions the d-transition metals are absolutely insoluble in carbon and at low metal concentrations do not react with carbon in the solid phase [1,2]. It is to be noted that the attempts made earlier to modify the electrocatalytic properties of carbon materials B-91 by use of ion ~plantation (without ion mixing) most commonly did not result in a considerable change in activity. The mechanism of the ion implantation effect on the catalytic properties of solid surfaces has not been explained so far. The researchers have established only the predominant role of specific process effects stimulated by the nature of the ion [lo-131.
2. Experimental The ion implantation was carried out at an accelerating voltage of 10 kV in the range of 6 X lOI2 to 1.7 X 1Or6 ions cme2. The content of implanted metals in the surface area was calculated using Rutherford backscattering spectroscopic data on He” ions. Peak metal concentrations were not higher than 10 at.%. A study of the electrocatalytic activity of modified materials was carried out for the electrochemical hydrogen evolution from 1M H,S04 at room temperature. The solution was prepared by using reagent grade chemicals and redistilled water. The hydrogen evolution curves obtained by cathode polarization were taken under potentiokinetic conditions at a potential sweep
535
V, V. Poplavskij / Electrocatalytic activity of transition-metal implanted carbon rate of 0.4 mV/s. A saturated silver chloride electrode (SSCE) is used as a reference electrode. The preliminary cleaning of the electrodes studied was done by means of holding them for 30 min at a cathode potential of - 0.4 V. The solution was neither stirred nor de-aerated. It was taken into account that the maximum diffusion current density for oxygen reduction under the experimental conditions had been determined both through calculation [14] and experimentally [8] to account for only 5 0.1% of the cell current density and to be negligible with respect to the hydrogen evolution involved. The dependence of electrode electrochemical behavior upon implanted metal concentration on a surface area which is changing due to corrosion during voltammetric measurements has been studied. An example of such an investigation is given in sect. 3.1 where surface adsorption properties of carbon materials alloyed with silver ions are discussed. However, the dependence only holds qualitatively, for the electrode processes concerned have been seen to penetrate to a considerable depth in the case of compact carbon materials (see ref. [15]), with a substantial amount of modified surface area working. The electron structure features of implanted atoms were studied by two independent methods: voltammetry in 1M H,SO, and X-ray photoelectron spectroscopy (ES2401, USSR). The emission of photoelectrons was excited by Mg K, radiation under a pressure of lop7 Pa. Photoelectronic spectra where obtained for samples held for several months after having been implanted at room temperature both without and with preliminary cleaning by means of etching for 30 min with Ar+ ions of 800 eV energy, the ionic current density being 80 PA cmW2.
3. Results
? .l-
0 L
vs.
.8 SSCf
v
Fig. 1. Cyclic voltammograms for electrodes of unimplanted glassy carbon (A), glassy carbon implanted with argon ions, 70 keV, 5.6 x lOI cmm2 (B), and silver ions, 1.4X 1015 cm-* (C) in 1M H,SO,. The potential sweep rate is 40 mV/s.
and discussion
3. I. Electrochemical
properties
A high adsorptivity relative to hydrogen has been found to be a distinctive feature of an ion-alloyed surface. It was illustrated that cyclic voltammograms for carbon materials alloyed with silver ions have current peaks specified by hydrogen adsorption and desorption (figs. 1 and 2) although the silver itself does not chemisorb the hydrogen [16]. The peaks appear after the hydrogen evolution polarization curves have been recorded. The peak intensity increases during the cycling, with electrode adsorption properties being activated due to greater active surface area involvement. During the cycling, done with successively greater anode potentials, the ionization current peak intensity for desorbing hydrogen first increases and later, as the electrode surface dissolves, decreases. As a result of complete depletion of
Fig. 2. Cyclic voltammograms for electrodes of unimplanted graphite (A), and graphite implanted with silver ions, 1.5 x 1015 cmm2 (B) in 1M H,SO,. The potential sweep rate is 40 mV/s.
536
V. V. Poplavskij / Electrocatalytic activity of transition-metal implanted carbon
the modified layer cyclic voltammograms are like those for unmodified carbon materials having no peaks due to adsorbed hydrogen. This behavior corresponds to the implanted metal distribution profile. Due to a high adsorptivity, the discharge process of hydrogen ions on the electrode surface is facilitated, resulting in a decrease of the hydrogen overpotential. This an be visualized by cathodic polarization curves (figs. 3 and 4) taken in the region of high current densities j, which are of interest for commercial electrolysis. The electrocatalytic surface activity for j 2 3 kA/m2 substantially exceeds that of an unmodified carbon surface. As the current density increases the adsorbed hydrogen gradually accumulates on the material surface, which hinders the movement of hydrogen ions from the solution to the cathode, causing an increase in overpotential. The two effects are especially strong on glassy carbon alloyed with iron ions (fig. 3f), the latter being strong also in the case of glassy carbon alloyed with titanium ions (fig. 3g). The adsorptivity increases with an increasing implanted metal content. Keeping the samples for a long time at room temperature has been found to lead to a progressive change in
3
2
r
i -bq
0 1
.5 -E,s
RHE/V
Fig. 4. Cathodic polarization curves of the hydrogen evolution from 1M H,SO, at 25°C on graphite electrodes. Use was made of electrodes implanted with ions of (A) platinum, 1.5 X 1015 cm-2; (B) vanadium, 1.6 X 10” cmW2; (C) iridium, 8.5 X 1014 cmm2; (D) unimplanted graphite. The curves were taken (a) immediately after ion implantation and (b) after ageing at room temperature. The potential sweep rate is 0.4 mV/s.
their properties. The surface activity of the materials under investigation after recovery correlates with that of transition metals and in most cases remains high as well. In this case it increases considerably for the region of high current densities in the cell (see fig. 4). 3.2. Electron structure of implanted atoms and its features
.s
--E”S
1.8
R”:,”
Fig. 3. Cathodic polarization curves of the hydrogen evolution from 1M H,SO, at 25OC on glassy carbon electrodes. Use was made of electrodes implanted with ions of (A) silver, 1.8 X lOI cm- ‘; (B) vanadium, 1.2 x 1015 cm -2; (C) platinum, 1.3 ~10’~ cmM2; (E) nickel, 2.1 X lOI cm- 2; (D) tungsten, 1.1 x 1016 cm-‘; (F) iron, 7.9 X 10” cmm2; and (G) titanium, 1.6 x 1016 cmm2; (H) unimplanted glassy carbon. The potential sweep rate is 0.4 mV/s.
The electron structure studies of sample surfaces showed that the implanted atoms of transition metals are in metastable states exhibiting an unexpected high extent of unoccupancy of valence electron levels. In the photoelectron spectra excited from the core levels of implanted metal atoms, along with spectral lines corresponding to the metal state, there are lines of comparable intensity with a higher binding energy, indicating that part of the atoms have unoccupied valence electron levels (figs. 5-8). The energy shift, AE, is significant. For level Fe 2p, its value amounts, e.g., to 4.8 eV; for Ni 2p, it is 5.6 eV; for Pt 4d, 4.0 eV. The experiments on anodic oxidation of ion-alloyed glassy carbon surfaces revealed a considerable increase (- 1 v) in the oxidation potential of implanted metals (figs. 9 and lo), which is characteristic for the oxidation of implanted atoms in the form of high-valence ions.
V. V. Poplavskij I
I
/ Electroeatalytic
activity of transition-metal
531
implanted carbon
I
I
AE Fe 2~72
fi
Fe 2p 3/2 I
I
700 hdrng
I
720
I
I 740
energy/ev
Fig. 5. Spectrum of photoelectrons excited from the 2p level of iron atoms implanted into glassy carbon. The ion dose is 9.0X 1015 cmm2. ES2401 spectrometer; X-rays, Mg K,, 1253.6 eV.
The analysis of the results obtained and their comparison with literature data permit to conclude that the effective valency of implanted transition metal atoms into a carbon matrix is higher than in the case of carbide and other known systems that incorporate a transition metal and carbon. The chemical shift, for example, for transition metal core levels in carbides is as low as 2 eV [17,18]; the Pt 4f level shift in the PtC, phase obtained by means of platinum sputtering in a carbonaceous atmosphere is s 0.16 eV [19]. The energy shift for core levels of metals deposited in submonolayer amounts at the surface of inert materials, including carbon ones, is as low as ca. 1.5 eV, with the shift amount being dependent on the extent of the coverage of the deposited metal and cluster size [20-241 (see also fig. 11). In the system studied, two spectral lines of the same energy level of implanted metal atoms have been recorded due to their equilibrium and metastable states respectively, with the energy shift being dependent on the nature of the metal.
850
870
890
Binding energy/eV Fig. 6. Spectrum of photoelectrons excited from the 2p level of nickel atoms implanted into glassy carbon. The ion dose is 1.6 X 1016 cme2. ES2401 spectrometer; X-rays, Mg K,, 1253.6 eV.
I 310
Binding
I
I
I
330
energy/Y
Fig. 7. Spectra of photoelectrons excited from the 4d level of platinum atoms implanted into glassy carbon: (A) after implantation and keeping at room temperature; (B) after annealing of the studied sample at 300 o C within 2 h. The ion dose is 1.5 X 1016 cm-‘. ES2401 spectrometer; X-rays, Mg K,, 1253.6 eV.
The implanted system gradually changes to an equilibrium state as a result of diffusion of metal atoms and precipitation as a consequence of recovery at room temperature. The precipitation can be activated by thermal annealing. For instance, annealing at 300 o C within 2 h considerably decreases the intensity of spectral lines corresponding to the platinum atoms implanted into glassy carbon with highly unoccupied electron states (fig. 7). An analysis of the electronic configuration of carbon and oxygen atoms as well as of components of the modified surface has revealed the formation of metastable states of implanted metallic atoms due to local electronic interaction with the carbon matrix. Thus, implantation results in the formation of metastable states of transition metal atoms implanted into the carbon matrix that are characterized by unexpected unoccupied valence electron levels. Upon implantation of insoluble impurity atoms, strong crystalline local fields are developed which cause not only variations in the configuration of electron levels of transition metal atoms observed earlier [25,26] but also a delocalization of their valence electrons. The implanted metal atoms exhibiting highly unoccupied d-electron levels are associated with coordinately unsaturated sites of an electron exchange in the material surface layer. This is the reason for an increase in adsorptivity and the characteristics observed in electrocatalytic hydrogen evolution. Upon the system’s transition to an equilibrium state the active chemisorption sites appear to be associated with metal precipitates and the surface properties of carbon materials correlate with the properties of modifying metals. The bonding strength of the adsorbed hydrogen
mea Bi
295.118
ndi ng Energy
3% UB
315.86
CeV)
Fig- 8. Spectrum of photoelectrons excited from the Is levef of carbon atoms and the 4d level of iridium atoms in the mrface area of &SSy carbon ~p~~n~~ with iridium ions, The ioix dose is 1.7 X lox6 cm --2. VG ESCAIAE-5 spectrometer; X-rays, Al K,, 14X6.6 eV.
t
Fig. 9. PotentM variation in the armdic oxidation of iron atoms implanted into glassy carbon. The ion dose is 7.9 X lOI cm-‘. 1M F$,SO,. The potential sweep rate is 40 mV/s.
’
I
I
I
I 1
Fig. 10. Potential variation in the anodic oxidatiozx of nickel atoms implanted into glassy carbon. The ion dose is 2.1 X 1Oi6 cm-*. 1M H,SO,. The potential sweep rate is 40 mV/s.
V. V. Poplavskij
/ Electrocatalytic
activity of transition-metal
implanted carbon
[31 M. Voinov, D. Bier
310
Binding
330
energy
/ eV
Fig. 11. Spectrum of photoelectrons excited from the 4d level of platinum atoms deposited on a glassy carbon surface. 2 X lOi -2; 0.05M H,PtCl, .6H,O + 0.02% Pb(C,H,O,), ;:(OH) s solution. ES2401 spectrometer; X-rays, Mg K, 1253.6 eV.
at the modified surface being different, hydrogen evolution reaction mechanisms also differ. Despite the difference in the nature of cathode processes, a high catalytic activity of the modified electrodes is retained in both cases. The author would like to thank A.A. Ivko and P.D. Zhuk for assistance in performing the X-ray photoelectron spectroscopy experiments.
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539
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