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Electrochemical behaviour of electrodeposited nanostructured palladium+platinum films in 2 M H2SO4
Electrochemistry Communications 3 (2001) 544±548
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Electrochemical behaviour of electrodeposited nanostructured palladium platinum ®lms in 2 M H2SO4 Samuel Guerin *, George S. Attard Department of Chemistry, University of Southampton, Southampton, SO17 1BJ, UK Received 23 April 2001; received in revised form 9 July 2001
Abstract Nanostructured palladium platinum ®lms were electrodeposited from an aqueous binary mixture that contained a non-ionic surfactant to produce a liquid crystalline phase. This phase acts as a template for the formation of mesoporous nanostructured ®lms. The ®lms produced were characterised by X-ray diraction and transmission electron microscopy in order to assess the existence of a regular nanostructure. Furthermore, cyclic voltammetry in 2 M H2 SO4 was used to obtain information about the electrochemical properties of the ®lms. Here we report the existence of two unexpected very sharp peaks in the hydrogen region that were observed by cyclic voltammetry. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Palladium platinum alloys; Nanostructure; Cyclic voltammetry
1. Introduction
2. Experimental
The properties of Pt group metals have made them an invaluable source of systems/materials with a wide range of application. In addition, their electrochemical properties have been exploited in studies of chemisorption phenomena at electrodes made of those metals [1,2]. Recently, we have shown that it is possible to produce mesoporous nanostructured platinum or palladium ®lms electrochemically using a non-ionic surfactant which acts as a template/mould when mixed with an aqueous solution of the dissolved metal salts [3±5]. Furthermore we showed that the mesoporous palladium, can be used as catalytic coatings in methane sensors [3]. Here we report the production and characterisation of nanostructured palladium platinum ®lms co-deposited electrochemically from a lyotropic liquid crystalline arrangement. Although this material is expected to be of some interest for catalysis, we have observed an unusual electrochemical phenomenon that is absent from non-templated systems.
The solutions used for deposition were made of 50 wt% of an aqueous solution of 40 mM ammonium tetrachloropalladate ( NH4 2 PdCl4 , Aldrich) 40 mM ammonium tetrachloroplatinate ( NH4 2 PtCl4 , Aldrich) and 50 wt% octaethyleneglycol monohexadecyl ether (C16 EO8 , Jan Dekker UK). All glassware was soaked overnight in distilled water with Decon (Aldrich), rinsed with distilled water and dried in a glass oven. The water used to make up the solutions was triply distilled, while the water used to clean and rinse the glassware was doubly distilled, both puri®ed with an Elga Purelab system. Electrochemistry was performed using a three electrode setup. Working electrodes were in the form of either gold disc electrodes (made from a gold wire of known diameter embedded in epoxy resin) or evaporated gold on microscope glass slides depending on subsequent experiments to be carried out. Gold disc electrodes were polished using sandpaper, 25, 1 and then 0.3 lm alumina. Gold on glass electrodes were cleaned by dipping into isopropanol in an ultrasonic bath for 30 min. Platinum gauze was used as counter electrode. The reference electrode used was a home-made saturated calomel electrode (SCE), whose potential was regularly checked against a commercial SCE (Cole and Parmer).
*
Corresponding author. Fax: +44-2380-593781. E-mail address: [email protected] (S. Guerin).
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 1 ) 0 0 2 1 1 - 9
S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
545
A potential dierence of 3 mV was tolerated between the commercial and the home-made SCE. All potentials are quoted relatively to the SCE unless otherwise stated. Transmission electron microscopy (TEM) was performed using a JEOL-JEM 2000 FX. X-ray diraction (XRD) deposited ®lms was performed using a Siemens H±2H diractometer. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed using a JEOL-JFM 6400. 3. Results and discussion For nomenclature, we use for the metal ®lms as follows: HI ±e Pd Pt refers to electrodeposited nanostructured Pd Pt ®lms, HI ±ePd and HI ±ePt to electrodeposited nanostructured Pd and electrodeposited nanostructured Pt, respectively (the pre®x HI denotes the liquid crystalline phase used in the electrodeposition). Pd and Pt ®lms electrodeposited from aqueous solutions are referred to as ePd and ePt while e(Pd Pt) refers to Pd Pt electrodeposited from aqueous solution. Nanostructured palladium platinum ®lms (HI ±e Pd Pt) were produced by applying a potential of 0.2 V between the working electrode and the reference electrode. The amount of palladium and platinum deposited was controlled by the charge passed during deposition. It is noted that this charge can only give an estimate of the amount of palladium and platinum because of uncertainty in the faradaic eciency of the deposition. The deposited ®lms were left in deionised water with forced convection, for on average 18 h, in order to remove the surfactant from the pores. The long ranged mesoporosity of the HI ±e Pd Pt ®lms was investigated using low angle XRD, while TEM was employed to obtain direct evidence of the existence of a nanostructure. Low angle XRD was performed between 1° and 5° in 2H and two peaks were observed at 1.7° and 3.03°. These are assigned to diraction from a 2D hexagonal lattice, with a lattice parameter of 5:92 0:1 nm. Direct evidence of the existence of a nanostructure as the result of the deposition of HI ±e Pd Pt was obtained by TEM. Fig. 1 shows a micrograph of a HI ±e Pd Pt ®lm electrodeposited at 0.2 V on evaporated gold on glass. The parallel pores running through the material are clearly visible. The periodicity of the structure can be calculated directly from the micrograph. A lattice parameter of 5.8 nm was found which in agreement with the value obtained from XRD. A pore diameter of 2.5 nm and a wall thickness of 2.5 nm was also estimated from the micrograph. The coexistence of both metals within the HI ±e Pd Pt ®lm was determined by EDS. For ®lms deposited at 0.2 V with no further treatment (other than
Fig. 1. TEM micrograph of HI ±e Pd Pt ®lm deposited at 0.2 V on evaporated gold on glass slide.
washing out the excess surfactant in deionised water) the composition was found to be: 62:07 3:57 at.%. of palladium and 38:67 3:40 at.%. of platinum; these values represent averages from ®ve samples and 11 measurements. No evidence of segregation of either metals has been observed on the scale of the resolution of the SEM indicating that if there is segregation it occurs in domains of less than 1 lm in size. Furthermore because of the complete miscibility of the two metal across the entire range of composition it is expected that an alloy (or true solid solution) should be formed [6]. Following removal of the surfactant the HI ±e Pd Pt ®lms were cycled in 2 M H2 SO4 between 0.25 and 1.4 V at 200 mV s 1 . Fig. 2 shows typical cyclic voltammograms of the HI ±e Pd Pt ®lms after 30 cycles. Each cyclic voltammogram is characterised by three distinctive regions, the double layer region, the oxide region and the hydrogen region. The double layer region lies between the hydrogen and the oxide regions.
Fig. 2. Typical cyclic voltammograms of HI ±e Pd Pt ®lms in 2 M H2 SO4 at 200 mV s 1 ; ®lm deposited on 1 mm diameter gold disc electrode at 0.2 V. Showing the new (Hred2 and Hox2 ) and standard ( Hred1 and Hox1 ) hydrogen peaks.
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S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
For the HI ±e Pd Pt ®lms the double layer region can be observed between 0.1 and 0.6 V in the anodic scan and between 0.1 and 0.25 V in the cathodic scan. The oxide region starts at 0.6 V in the anodic scan and ®nishes in the cathodic scan at 0.3 V. Two main features can be observed: an extended oxidation peak in the anodic scan associated with oxygen adsorption above 0.6 V and a sharp reduction peak in the cathodic scan corresponding to oxygen stripping at 0.45 V. The hydrogen region starts at 0.1 V in the cathodic scan and ®nishes at 0.1 V in the anodic scan. Several features can be observed: ®rstly, in the cathodic scan, two reduction peaks of similar magnitude and shape, the ®rst one at 0.015 V and the second one at 0.098 V, then a very sharp reduction peak at 0.240 V. Secondly, in the anodic scan, a very sharp oxidation peak at 0.146 V and then two smaller oxidation peaks at 0.068 and 0.014 V. Comparison can be drawn between HI ±ePd and HI ±ePt electrodes and the HI ±e Pd Pt electrode. The features occurring in the oxide region (i.e. oxide formation and reduction) are similar to those of palladium and platinum electrodes. In the hydrogen region three peaks were observed in the cathodic scan and also in the anodic scan. Due to their shape, intensities and therefore their charge, it seems logical to associate the reduction peaks at 0.01, 0.10 and 0.24 V with, respectively, the oxidation peaks at 0.01, 0.07 and 0.15 V. In the case of both HI ±ePt and ePt, four peaks can be observed in the hydrogen region at 0.01 and 0.12 V for the reduction peaks and 0.09 and 0.04 V for the oxidation peaks; these peaks are associated with weak and strong adsorption/desorption of hydrogen on platinum [2]. It can be noticed that the four ``weak'' peaks recorded on HI ±e Pd Pt do not correspond exactly to those observed on HI ±ePt; in the case of HI ±e Pd Pt the peaks are closer in potential; those four ``weak'' peaks will be referred in this paper as normal peaks. As for the very sharp peaks at 0.24 and 0.15 V they do not seem to correspond to any of the common features of either metal when cycled in sulfuric acid; these peaks will be referred in the present paper as new peaks. The hydrogen region in case of palladium does not exhibit such features. For Palladium disc electrodes (or ePd electrodes) the hydrogen region only shows features which are associated with hydrogen absorption into the palladium bulk. Furthermore this usually happens at a potential where hydrogen evolution takes place leading to a massive reduction current in the cathodic scan. Desorption of hydrogen from the palladium bulk happens in the anodic scan and gives rise to a very broad and intense peak which makes all other surface-induced features appear very small [2]. In the case of HI ±ePd electrodes some dierences, when comparing to ePd or Pd electrodes, can be noticed. It was reported that two peaks at 0.04 and 0.11 V can be observed in, re-
spectively, the cathodic and anodic scan [3]. However these peaks dier in shape, position and intensity to the two peaks at 0.24 and 0.15 V recorded on HI ±e Pd Pt electrodes. In summary, the cyclic voltammograms of HI ±e Pd Pt electrodes in sulfuric acid show features associated with oxygen adsorption and desorption on both the constituent metals and probably those of hydrogen adsorption and desorption on platinum. However no features corresponding to known hydrogen interactions with either Pd, ePd or HI ± ePd were observed. Instead two very sharp and intense peaks in the hydrogen region were observed. In order to investigate the origin and the nature of these new peaks several experiments were carried out. Fig. 3 shows the behaviour of the HI ±e Pd Pt electrode over the ®rst 12 cycles. This behaviour has been found to be typical of all HI ±e Pd Pt ®lms produced with this method. It can be noticed that the new peaks are absent in the ®rst cycle and only appear on cycling. Furthermore Fig. 3 shows that the oxide stripping peak displays evidence of two peaks, or a shoulder, before it stabilises at the position shown in Fig. 2. Concurrent with the changes in the oxygen stripping peak, changes in the oxide formation peak and in the normal hydrogen peaks can be observed. It was found that in order to make the new peaks appear, the HI ±e Pd Pt electrodes had to be cycled within the limits of the oxygen region. Continuous cycling in the hydrogen and double layer regions did not result in the formation of the new peaks. From the cyclic voltammogram shown in Fig. 3 it can be noticed that the oxygen stripping peak reaches its maximum amplitude before the ``new'' peaks do. This would appear to rule out the possibility that the ``new'' peaks are due exclusively to a cleaning or re-organisation process at the surface of the electrode.
Fig. 3. First 12 cyclic voltammograms of a ``fresh'' HI ±e Pd Pt ®lm in 2 M H2 SO4 at 200 mV s 1 ; ®lm deposited on 1 mm diameter gold disc electrode at 0.2 V.
S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
Cyclic voltammetry experiments were carried out when the HI ±e Pd Pt electrodes were in the state shown in Fig. 2 (i.e. all features remained stable while cycling). The maxima of the various hydrogen peaks present in the cyclic voltammogram were recorded as the scan rate was varied from 1 to 1000 mV s 1 . Fig. 4 shows a double logarithmic plot of the data from these experiments. It is clear from this graph that the new peaks exhibit a dierent dependence on scan rate than the normal peaks. The variation of the maxima of the normal hydrogen peaks follows a straight line throughout the range of scan rates, whereas the variation of the maxima of the new peaks shows two different slopes. At lower scan rates the slopes observed are similar in the two systems. However at higher scan rates a lower slope can be observed for the new hydrogen peaks. The slopes of the various processes were calculated from Fig. 4 using linear ®ts. In all cases a correlation coecient higher than 0.996 was obtained indicating relatively good ®ts. The numerical results are summarised in Table 1. The point at which the new peaks in HI ±e Pd Pt deviate from the behaviour observed for the normal peaks was found to be between 20 and 100 mV s 1 . The slope at low scan rates was found to be close to unity, suggesting a pure
Fig. 4. Variation of the log of the maxima of the new (Hred2 and Hox2 ) and standard (Hred1 and Hox1 ) hydrogen peaks with the log of the scan rate. The scan rate was varied from 1 to 1000 mV s 1 . Experiments carried out in 2 M H2 SO4 .
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surface process. At higher scan rates the slope was found to be around 0.65. A pure diusion controlled process should give a slope of 0.5, since in such case the intensity of the peak varies linearly with the square root of the scan rate. Therefore it seems that the process happening at high scan rate in the case of the new hydrogen peaks is a mixed surface process and diusion controlled process. 3.1. Proposed model Of the two metals in HI ±e Pd Pt platinum is more noble. It is also known that palladium dissolves in sulfuric acid at a much higher rate than platinum [7]. Dissolution of palladium while cycling in sulfuric acid happens as oxide formation occurs due to a complex formation between the Pd ions and the sulfate anions [8,9]. Furthermore it has been reported that in the case of HI ±ePd, replating of palladium happens with the reduction of the oxide [3]. In the light of this it is possible that the new peaks are a consequence of this dissolution and replating eect. During cyclic voltammetry some palladium will be stripped from the electrode surface and replated leading to the formation, after several cycles, of islands or layers of Pd. These layers or islands, as they get large enough, will start to show the standard behaviour for hydrogen absorption/desorption on and into palladium. This process is not a surface process which can account for the large currents recorded. The change of slope observed for these new peaks ®ts well with the formation of islands/layers of palladium. If those islands are of small size there will be a limitation to the amount of hydrogen that can be absorbed. The hydrogen is absorbed at 0.24 V and the amount of hydrogen absorbed will depend on the time spent in the hydrogen region. Therefore it seems reasonable to assume that at slow scan rates there is enough time to fully load the Pd particles with hydrogen whereas at faster scan rates there is not. The net eect of such a phenomenon would be to make the absorption at slow scan rates appear like a surface process. At faster scan rates, when hydrogen does not have enough time to fully load the Pd particles, the limiting factor would be the diusion of hydrogen into the Pd.
Table 1 Calculated values of the slopes of the various processes shown in Fig. 4 Peaks
Slope
Error
r
No. of points
Hred1 Hox1 Hred2 slow scan rate Hox2 slow scan rate Hred2 fast scan rate Hox2 fast scan rate
0.85 0.95 0.90 0.89 0.66 0.63
0.01 0.01 0.04 0.05 0.01 0.01
0.999 0.999 0.997 0.996 0.999 0.999
11 11 5 5 5 5
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S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
We also found that the new peaks could be observed on a HI ±ePt electrode on which palladium had been electrodeposited from an aqueous phase. However the new peaks have never been recorded in the case of e(Pd Pt) electrodes suggesting that the presence of nanostructure is required to observe such phenomenon. Very sharp and intense peaks similar to those observed on our samples were reported by Attard and Clavilier for Pd irreversibly adsorbed onto Pt single crystals [10±12]. These abnormal peaks were attributed to adsorption states of hydrogen onto the Pd particles. The possible absorption of hydrogen into the Pd was mentioned for Pd adatoms on Pt(1 1 1) although not supported by these authors [12]. Instead they found that their sharp peak was dependent on the strength of anion adsorption. These peaks were very sharp and intense in sulfuric acid electrolyte however they were much broader in perchloric acid. In order to investigate whether anion adsorption had any in¯uence on our ``new'' peaks, cyclic voltammetry experiments in perchloric acid and sodium hydroxide electrolytes were performed. The scan rate was varied from 1 to 1000 mV s 1 and the intensity of the peaks was recorded in order to produce a double logarithmic plot of current vs. scan rate. The cyclic voltammograms in perchloric acid are identical to those recorded in sulfuric acid and those in sodium hydroxide are similar. The double logarithmic plot obtained from the cyclic voltammograms in perchloric acid gave identical behaviour to sulfuric acid (i.e. a slope of 0.9 at low scan rates and a slope of 0.66 at higher scan rates, with the same turning point). 4. Conclusion We have reported the production of a novel type of electrode material, HI ±e Pd Pt, produced by simultaneous electrodeposition of both palladium and platinum onto gold substrates from a gel-like solution containing both metal salts dissolved in water and C16 EO8 , a non-ionic surfactant which acts as a tem-
plate for the formation of the nanostructure. The material produced this way was found to have a composition of 62.07 at.% of Pd and 38.67 at.% of Pt. XRD and TEM have shown the existence of nanostructure while cyclic voltammetry in sulfuric acid has led to the apparition of unexpected very sharp and intense peaks in the hydrogen region. These peaks are believed to be a consequence of the dissolution and replating of the palladium during cycling leading to the formation of islands of palladium. These islands enable hydrogen absorption into the palladium giving rise to the very sharp peaks observed. Acknowledgements The authors would like to acknowledge R. Parsons and D. Pletcher for useful discussions and the EPSRC for ®nancial support.
References [1] P.J. Mitchell, N.A. Hampson, A.J.S. McNeil, in: R.S.C. (Ed.), Electrochemistry, vol. 10, R.S.C., London, 1985. [2] R. Woods, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 9, Marcel Dekker Inc., New York-Basel, 1976. [3] S. Guerin, Ph.D. thesis, Department of Chemistry, University of Southampton, Southampton, 1999. [4] G.S. Attard, P.N. Bartlett, N.R.B. Coleman, J.M. Elliot, J.R. Owen, J.H. Wang, Science 278 (1997) 838. [5] J.M. Elliott, G.S. Attard, P.N. Bartlett, N.R.B. Coleman, D.A.S. Merckel, J.R. Owen, Chem. Matter. 11 (1999) 3602. [6] H. Baker (Ed.), ASM Handbook, vol. 3, ASM International, 1991, p. 337. [7] D.A.J. Rand, R. Woods, J. Electroanal. Chem. 35 (1972) 209. [8] J.N. Crosby, J.A. Harrison, T.A. Whit®eld, Electrochim. Acta 26 (1981) 1647. [9] A.E. Bolzan, M.E. Martins, A.J. Arvia, J. Electroanal. Chem. 172 (1984) 221. [10] G.A. Attard, A. Bannister, J. Electroanal. Chem. 300 (1991) 467. [11] J. Clavilier, M.J. Llorca, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 310 (1991) 429. [12] M.J. Llorca, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 351 (1993) 299.
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Electrochemical behaviour of electrodeposited nanostructured palladium platinum ®lms in 2 M H2SO4 Samuel Guerin *, George S. Attard Department of Chemistry, University of Southampton, Southampton, SO17 1BJ, UK Received 23 April 2001; received in revised form 9 July 2001
Abstract Nanostructured palladium platinum ®lms were electrodeposited from an aqueous binary mixture that contained a non-ionic surfactant to produce a liquid crystalline phase. This phase acts as a template for the formation of mesoporous nanostructured ®lms. The ®lms produced were characterised by X-ray diraction and transmission electron microscopy in order to assess the existence of a regular nanostructure. Furthermore, cyclic voltammetry in 2 M H2 SO4 was used to obtain information about the electrochemical properties of the ®lms. Here we report the existence of two unexpected very sharp peaks in the hydrogen region that were observed by cyclic voltammetry. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Palladium platinum alloys; Nanostructure; Cyclic voltammetry
1. Introduction
2. Experimental
The properties of Pt group metals have made them an invaluable source of systems/materials with a wide range of application. In addition, their electrochemical properties have been exploited in studies of chemisorption phenomena at electrodes made of those metals [1,2]. Recently, we have shown that it is possible to produce mesoporous nanostructured platinum or palladium ®lms electrochemically using a non-ionic surfactant which acts as a template/mould when mixed with an aqueous solution of the dissolved metal salts [3±5]. Furthermore we showed that the mesoporous palladium, can be used as catalytic coatings in methane sensors [3]. Here we report the production and characterisation of nanostructured palladium platinum ®lms co-deposited electrochemically from a lyotropic liquid crystalline arrangement. Although this material is expected to be of some interest for catalysis, we have observed an unusual electrochemical phenomenon that is absent from non-templated systems.
The solutions used for deposition were made of 50 wt% of an aqueous solution of 40 mM ammonium tetrachloropalladate ( NH4 2 PdCl4 , Aldrich) 40 mM ammonium tetrachloroplatinate ( NH4 2 PtCl4 , Aldrich) and 50 wt% octaethyleneglycol monohexadecyl ether (C16 EO8 , Jan Dekker UK). All glassware was soaked overnight in distilled water with Decon (Aldrich), rinsed with distilled water and dried in a glass oven. The water used to make up the solutions was triply distilled, while the water used to clean and rinse the glassware was doubly distilled, both puri®ed with an Elga Purelab system. Electrochemistry was performed using a three electrode setup. Working electrodes were in the form of either gold disc electrodes (made from a gold wire of known diameter embedded in epoxy resin) or evaporated gold on microscope glass slides depending on subsequent experiments to be carried out. Gold disc electrodes were polished using sandpaper, 25, 1 and then 0.3 lm alumina. Gold on glass electrodes were cleaned by dipping into isopropanol in an ultrasonic bath for 30 min. Platinum gauze was used as counter electrode. The reference electrode used was a home-made saturated calomel electrode (SCE), whose potential was regularly checked against a commercial SCE (Cole and Parmer).
*
Corresponding author. Fax: +44-2380-593781. E-mail address: [email protected] (S. Guerin).
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 1 ) 0 0 2 1 1 - 9
S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
545
A potential dierence of 3 mV was tolerated between the commercial and the home-made SCE. All potentials are quoted relatively to the SCE unless otherwise stated. Transmission electron microscopy (TEM) was performed using a JEOL-JEM 2000 FX. X-ray diraction (XRD) deposited ®lms was performed using a Siemens H±2H diractometer. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed using a JEOL-JFM 6400. 3. Results and discussion For nomenclature, we use for the metal ®lms as follows: HI ±e Pd Pt refers to electrodeposited nanostructured Pd Pt ®lms, HI ±ePd and HI ±ePt to electrodeposited nanostructured Pd and electrodeposited nanostructured Pt, respectively (the pre®x HI denotes the liquid crystalline phase used in the electrodeposition). Pd and Pt ®lms electrodeposited from aqueous solutions are referred to as ePd and ePt while e(Pd Pt) refers to Pd Pt electrodeposited from aqueous solution. Nanostructured palladium platinum ®lms (HI ±e Pd Pt) were produced by applying a potential of 0.2 V between the working electrode and the reference electrode. The amount of palladium and platinum deposited was controlled by the charge passed during deposition. It is noted that this charge can only give an estimate of the amount of palladium and platinum because of uncertainty in the faradaic eciency of the deposition. The deposited ®lms were left in deionised water with forced convection, for on average 18 h, in order to remove the surfactant from the pores. The long ranged mesoporosity of the HI ±e Pd Pt ®lms was investigated using low angle XRD, while TEM was employed to obtain direct evidence of the existence of a nanostructure. Low angle XRD was performed between 1° and 5° in 2H and two peaks were observed at 1.7° and 3.03°. These are assigned to diraction from a 2D hexagonal lattice, with a lattice parameter of 5:92 0:1 nm. Direct evidence of the existence of a nanostructure as the result of the deposition of HI ±e Pd Pt was obtained by TEM. Fig. 1 shows a micrograph of a HI ±e Pd Pt ®lm electrodeposited at 0.2 V on evaporated gold on glass. The parallel pores running through the material are clearly visible. The periodicity of the structure can be calculated directly from the micrograph. A lattice parameter of 5.8 nm was found which in agreement with the value obtained from XRD. A pore diameter of 2.5 nm and a wall thickness of 2.5 nm was also estimated from the micrograph. The coexistence of both metals within the HI ±e Pd Pt ®lm was determined by EDS. For ®lms deposited at 0.2 V with no further treatment (other than
Fig. 1. TEM micrograph of HI ±e Pd Pt ®lm deposited at 0.2 V on evaporated gold on glass slide.
washing out the excess surfactant in deionised water) the composition was found to be: 62:07 3:57 at.%. of palladium and 38:67 3:40 at.%. of platinum; these values represent averages from ®ve samples and 11 measurements. No evidence of segregation of either metals has been observed on the scale of the resolution of the SEM indicating that if there is segregation it occurs in domains of less than 1 lm in size. Furthermore because of the complete miscibility of the two metal across the entire range of composition it is expected that an alloy (or true solid solution) should be formed [6]. Following removal of the surfactant the HI ±e Pd Pt ®lms were cycled in 2 M H2 SO4 between 0.25 and 1.4 V at 200 mV s 1 . Fig. 2 shows typical cyclic voltammograms of the HI ±e Pd Pt ®lms after 30 cycles. Each cyclic voltammogram is characterised by three distinctive regions, the double layer region, the oxide region and the hydrogen region. The double layer region lies between the hydrogen and the oxide regions.
Fig. 2. Typical cyclic voltammograms of HI ±e Pd Pt ®lms in 2 M H2 SO4 at 200 mV s 1 ; ®lm deposited on 1 mm diameter gold disc electrode at 0.2 V. Showing the new (Hred2 and Hox2 ) and standard ( Hred1 and Hox1 ) hydrogen peaks.
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For the HI ±e Pd Pt ®lms the double layer region can be observed between 0.1 and 0.6 V in the anodic scan and between 0.1 and 0.25 V in the cathodic scan. The oxide region starts at 0.6 V in the anodic scan and ®nishes in the cathodic scan at 0.3 V. Two main features can be observed: an extended oxidation peak in the anodic scan associated with oxygen adsorption above 0.6 V and a sharp reduction peak in the cathodic scan corresponding to oxygen stripping at 0.45 V. The hydrogen region starts at 0.1 V in the cathodic scan and ®nishes at 0.1 V in the anodic scan. Several features can be observed: ®rstly, in the cathodic scan, two reduction peaks of similar magnitude and shape, the ®rst one at 0.015 V and the second one at 0.098 V, then a very sharp reduction peak at 0.240 V. Secondly, in the anodic scan, a very sharp oxidation peak at 0.146 V and then two smaller oxidation peaks at 0.068 and 0.014 V. Comparison can be drawn between HI ±ePd and HI ±ePt electrodes and the HI ±e Pd Pt electrode. The features occurring in the oxide region (i.e. oxide formation and reduction) are similar to those of palladium and platinum electrodes. In the hydrogen region three peaks were observed in the cathodic scan and also in the anodic scan. Due to their shape, intensities and therefore their charge, it seems logical to associate the reduction peaks at 0.01, 0.10 and 0.24 V with, respectively, the oxidation peaks at 0.01, 0.07 and 0.15 V. In the case of both HI ±ePt and ePt, four peaks can be observed in the hydrogen region at 0.01 and 0.12 V for the reduction peaks and 0.09 and 0.04 V for the oxidation peaks; these peaks are associated with weak and strong adsorption/desorption of hydrogen on platinum [2]. It can be noticed that the four ``weak'' peaks recorded on HI ±e Pd Pt do not correspond exactly to those observed on HI ±ePt; in the case of HI ±e Pd Pt the peaks are closer in potential; those four ``weak'' peaks will be referred in this paper as normal peaks. As for the very sharp peaks at 0.24 and 0.15 V they do not seem to correspond to any of the common features of either metal when cycled in sulfuric acid; these peaks will be referred in the present paper as new peaks. The hydrogen region in case of palladium does not exhibit such features. For Palladium disc electrodes (or ePd electrodes) the hydrogen region only shows features which are associated with hydrogen absorption into the palladium bulk. Furthermore this usually happens at a potential where hydrogen evolution takes place leading to a massive reduction current in the cathodic scan. Desorption of hydrogen from the palladium bulk happens in the anodic scan and gives rise to a very broad and intense peak which makes all other surface-induced features appear very small [2]. In the case of HI ±ePd electrodes some dierences, when comparing to ePd or Pd electrodes, can be noticed. It was reported that two peaks at 0.04 and 0.11 V can be observed in, re-
spectively, the cathodic and anodic scan [3]. However these peaks dier in shape, position and intensity to the two peaks at 0.24 and 0.15 V recorded on HI ±e Pd Pt electrodes. In summary, the cyclic voltammograms of HI ±e Pd Pt electrodes in sulfuric acid show features associated with oxygen adsorption and desorption on both the constituent metals and probably those of hydrogen adsorption and desorption on platinum. However no features corresponding to known hydrogen interactions with either Pd, ePd or HI ± ePd were observed. Instead two very sharp and intense peaks in the hydrogen region were observed. In order to investigate the origin and the nature of these new peaks several experiments were carried out. Fig. 3 shows the behaviour of the HI ±e Pd Pt electrode over the ®rst 12 cycles. This behaviour has been found to be typical of all HI ±e Pd Pt ®lms produced with this method. It can be noticed that the new peaks are absent in the ®rst cycle and only appear on cycling. Furthermore Fig. 3 shows that the oxide stripping peak displays evidence of two peaks, or a shoulder, before it stabilises at the position shown in Fig. 2. Concurrent with the changes in the oxygen stripping peak, changes in the oxide formation peak and in the normal hydrogen peaks can be observed. It was found that in order to make the new peaks appear, the HI ±e Pd Pt electrodes had to be cycled within the limits of the oxygen region. Continuous cycling in the hydrogen and double layer regions did not result in the formation of the new peaks. From the cyclic voltammogram shown in Fig. 3 it can be noticed that the oxygen stripping peak reaches its maximum amplitude before the ``new'' peaks do. This would appear to rule out the possibility that the ``new'' peaks are due exclusively to a cleaning or re-organisation process at the surface of the electrode.
Fig. 3. First 12 cyclic voltammograms of a ``fresh'' HI ±e Pd Pt ®lm in 2 M H2 SO4 at 200 mV s 1 ; ®lm deposited on 1 mm diameter gold disc electrode at 0.2 V.
S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
Cyclic voltammetry experiments were carried out when the HI ±e Pd Pt electrodes were in the state shown in Fig. 2 (i.e. all features remained stable while cycling). The maxima of the various hydrogen peaks present in the cyclic voltammogram were recorded as the scan rate was varied from 1 to 1000 mV s 1 . Fig. 4 shows a double logarithmic plot of the data from these experiments. It is clear from this graph that the new peaks exhibit a dierent dependence on scan rate than the normal peaks. The variation of the maxima of the normal hydrogen peaks follows a straight line throughout the range of scan rates, whereas the variation of the maxima of the new peaks shows two different slopes. At lower scan rates the slopes observed are similar in the two systems. However at higher scan rates a lower slope can be observed for the new hydrogen peaks. The slopes of the various processes were calculated from Fig. 4 using linear ®ts. In all cases a correlation coecient higher than 0.996 was obtained indicating relatively good ®ts. The numerical results are summarised in Table 1. The point at which the new peaks in HI ±e Pd Pt deviate from the behaviour observed for the normal peaks was found to be between 20 and 100 mV s 1 . The slope at low scan rates was found to be close to unity, suggesting a pure
Fig. 4. Variation of the log of the maxima of the new (Hred2 and Hox2 ) and standard (Hred1 and Hox1 ) hydrogen peaks with the log of the scan rate. The scan rate was varied from 1 to 1000 mV s 1 . Experiments carried out in 2 M H2 SO4 .
547
surface process. At higher scan rates the slope was found to be around 0.65. A pure diusion controlled process should give a slope of 0.5, since in such case the intensity of the peak varies linearly with the square root of the scan rate. Therefore it seems that the process happening at high scan rate in the case of the new hydrogen peaks is a mixed surface process and diusion controlled process. 3.1. Proposed model Of the two metals in HI ±e Pd Pt platinum is more noble. It is also known that palladium dissolves in sulfuric acid at a much higher rate than platinum [7]. Dissolution of palladium while cycling in sulfuric acid happens as oxide formation occurs due to a complex formation between the Pd ions and the sulfate anions [8,9]. Furthermore it has been reported that in the case of HI ±ePd, replating of palladium happens with the reduction of the oxide [3]. In the light of this it is possible that the new peaks are a consequence of this dissolution and replating eect. During cyclic voltammetry some palladium will be stripped from the electrode surface and replated leading to the formation, after several cycles, of islands or layers of Pd. These layers or islands, as they get large enough, will start to show the standard behaviour for hydrogen absorption/desorption on and into palladium. This process is not a surface process which can account for the large currents recorded. The change of slope observed for these new peaks ®ts well with the formation of islands/layers of palladium. If those islands are of small size there will be a limitation to the amount of hydrogen that can be absorbed. The hydrogen is absorbed at 0.24 V and the amount of hydrogen absorbed will depend on the time spent in the hydrogen region. Therefore it seems reasonable to assume that at slow scan rates there is enough time to fully load the Pd particles with hydrogen whereas at faster scan rates there is not. The net eect of such a phenomenon would be to make the absorption at slow scan rates appear like a surface process. At faster scan rates, when hydrogen does not have enough time to fully load the Pd particles, the limiting factor would be the diusion of hydrogen into the Pd.
Table 1 Calculated values of the slopes of the various processes shown in Fig. 4 Peaks
Slope
Error
r
No. of points
Hred1 Hox1 Hred2 slow scan rate Hox2 slow scan rate Hred2 fast scan rate Hox2 fast scan rate
0.85 0.95 0.90 0.89 0.66 0.63
0.01 0.01 0.04 0.05 0.01 0.01
0.999 0.999 0.997 0.996 0.999 0.999
11 11 5 5 5 5
548
S. Guerin, G.S. Attard / Electrochemistry Communications 3 (2001) 544±548
We also found that the new peaks could be observed on a HI ±ePt electrode on which palladium had been electrodeposited from an aqueous phase. However the new peaks have never been recorded in the case of e(Pd Pt) electrodes suggesting that the presence of nanostructure is required to observe such phenomenon. Very sharp and intense peaks similar to those observed on our samples were reported by Attard and Clavilier for Pd irreversibly adsorbed onto Pt single crystals [10±12]. These abnormal peaks were attributed to adsorption states of hydrogen onto the Pd particles. The possible absorption of hydrogen into the Pd was mentioned for Pd adatoms on Pt(1 1 1) although not supported by these authors [12]. Instead they found that their sharp peak was dependent on the strength of anion adsorption. These peaks were very sharp and intense in sulfuric acid electrolyte however they were much broader in perchloric acid. In order to investigate whether anion adsorption had any in¯uence on our ``new'' peaks, cyclic voltammetry experiments in perchloric acid and sodium hydroxide electrolytes were performed. The scan rate was varied from 1 to 1000 mV s 1 and the intensity of the peaks was recorded in order to produce a double logarithmic plot of current vs. scan rate. The cyclic voltammograms in perchloric acid are identical to those recorded in sulfuric acid and those in sodium hydroxide are similar. The double logarithmic plot obtained from the cyclic voltammograms in perchloric acid gave identical behaviour to sulfuric acid (i.e. a slope of 0.9 at low scan rates and a slope of 0.66 at higher scan rates, with the same turning point). 4. Conclusion We have reported the production of a novel type of electrode material, HI ±e Pd Pt, produced by simultaneous electrodeposition of both palladium and platinum onto gold substrates from a gel-like solution containing both metal salts dissolved in water and C16 EO8 , a non-ionic surfactant which acts as a tem-
plate for the formation of the nanostructure. The material produced this way was found to have a composition of 62.07 at.% of Pd and 38.67 at.% of Pt. XRD and TEM have shown the existence of nanostructure while cyclic voltammetry in sulfuric acid has led to the apparition of unexpected very sharp and intense peaks in the hydrogen region. These peaks are believed to be a consequence of the dissolution and replating of the palladium during cycling leading to the formation of islands of palladium. These islands enable hydrogen absorption into the palladium giving rise to the very sharp peaks observed. Acknowledgements The authors would like to acknowledge R. Parsons and D. Pletcher for useful discussions and the EPSRC for ®nancial support.
References [1] P.J. Mitchell, N.A. Hampson, A.J.S. McNeil, in: R.S.C. (Ed.), Electrochemistry, vol. 10, R.S.C., London, 1985. [2] R. Woods, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 9, Marcel Dekker Inc., New York-Basel, 1976. [3] S. Guerin, Ph.D. thesis, Department of Chemistry, University of Southampton, Southampton, 1999. [4] G.S. Attard, P.N. Bartlett, N.R.B. Coleman, J.M. Elliot, J.R. Owen, J.H. Wang, Science 278 (1997) 838. [5] J.M. Elliott, G.S. Attard, P.N. Bartlett, N.R.B. Coleman, D.A.S. Merckel, J.R. Owen, Chem. Matter. 11 (1999) 3602. [6] H. Baker (Ed.), ASM Handbook, vol. 3, ASM International, 1991, p. 337. [7] D.A.J. Rand, R. Woods, J. Electroanal. Chem. 35 (1972) 209. [8] J.N. Crosby, J.A. Harrison, T.A. Whit®eld, Electrochim. Acta 26 (1981) 1647. [9] A.E. Bolzan, M.E. Martins, A.J. Arvia, J. Electroanal. Chem. 172 (1984) 221. [10] G.A. Attard, A. Bannister, J. Electroanal. Chem. 300 (1991) 467. [11] J. Clavilier, M.J. Llorca, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 310 (1991) 429. [12] M.J. Llorca, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 351 (1993) 299.