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Electrochemical investigations of single microparticles
Electrochimica Acta 47 (2001) 117– 120 www.elsevier.com/locate/electacta
Electrochemical investigations of single microparticles T. Hamelmann *, M.M. Lohrengel Institut fu¨r Physikalische Chemie und Elektrochemie, Heinrich-Heine-Uni6ersita¨t Du¨sseldorf, Uni6ersita¨tsstraße 1, 40225 Dusseldorf, Germany Received 14 December 2000; received in revised form 13 March 2001
Abstract Small electroactive particles are important materials in many technical processes. A new method to investigate single micro particles with diameters in the range of 10 mm is presented. Small numbers of these particles are fixed on a gold sheet and investigated by a capillary-based droplet cell. Cyclovoltammograms of different samples (cementite or Ni(OH)2 particles) are presented as examples. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microcell; Single particle; Nickel hydroxide
1. Introduction Well-defined electrodes (like wires or sheets) cannot be formed from all electrochemically relevant materials. Examples are powders which are used as an active material in batteries or brittle materials which forms inclusions. Macroscopic investigations of pressed, sintered or embedded particles are fundamentally possible but involve large numbers of particles. If their ionic or electronic conductivity is poor, potential or concentration gradients may develop. Furthermore, the individual properties may differ from particle to particle, due to size, shape, structure, crystallinity, or composition and conventional experiments yield mean data only. Therefore, it is necessary to investigate single particles with the common potentiostatic techniques like cyclovoltammograms, transients or impedance. In this paper, we present a convenient method to prepare sheets with small numbers of particles with a size of some micrometer and show how they can be addressed individually by a special electrochemical microcell. Other techniques to measure single particles, e.g. by contacting particles embedded in insulating materials with carbon fibers [1], are limited due to the high contact resistance. * Corresponding author. E-mail address: [email protected] (T. Hamelmann).
Two examples are presented. Nickel hydroxide particles (2–20 mm in diameter) are a base material for rechargeable Ni/Cd- or Ni/MH-oxide or LiC/LiNiO2batteries. Their activity depends on the phase composition, shape, and inner structure of the particles. Particles of Fe3C are typical inclusions in steel and influence the mechanical properties, electrochemical machining [2], and corrosion.
2. Experimental For these investigations, we had to solve two problems: preparation of isolated particles with a welldefined electric contact and the local electrochemical investigation.
2.1. Preparation of samples Our fundamental idea was to fix small numbers of particles on a metal sheet so that the mean distance between the particles is much larger than their diameter. This should enable us to address individual particles with the scanning droplet cell. As a base sheet we used gold because this metal has the largest double layer region of all noble metals (approximately −0.2 to 1.4 V vs. SHE at pH 0). Therefore, an influence of this base should be small. Other materials with a large double layer region are liquid or non-metallic and, therefore cannot be galvanically deposited (discussed
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 5 5 6 - 4
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T. Hamelmann, M.M. Lohrengel / Electrochimica Acta 47 (2001) 117–120
Fig. 1. Schematic cross-section of single nickel hydroxide particles embedded in an electrodeposited gold layer.
later). Small amounts of particles were dispersed in pure water or organic solvents in an ultrasonic bath. The concentration of particles is very critical for the subsequent procedures. A defined small droplet of the suspension was pipetted on a gold sheet. After evaporation of the solvent, the particles adhered slightly to the gold sheet. The next step was to fix them irreversibly and to obtain a good electric contact with the gold sheet. This was done by galvanic embedding in gold, i.e. about 1.5 mm of gold was deposited (3 mA/cm2 for 500 s). This technique requires that no gold be deposited on the particles because the conductivity of the particles is small or if the formation of gold nuclei is hindered on the particle surface. Both may be true for the particles investigated here. This embedding procedure (Fig. 1) blocks parts of the particle surface typically in the range of 10% (assuming 1 mm of galvanic gold and 10 mm particles) but, on the other hand, guarantees a good contact (Fig. 2).
Fig. 3. Schematic presentation of single particles addressed with the droplet cell.
2.2. Electrochemical set-up A macroscopic investigation of a single particle on a gold sheet of e.g. 1 cm2 is impossible as side reactions of electroactive impurities from the electrolyte on the gold surface would suppress the response of the particle of some micrometers. Therefore, the electrochemical access had to be limited to a small region only around the particle. This becomes possible with the scanning droplet cell [3] (Fig. 3). An electrolyte droplet is dispensed and positioned on the surface by a capillary. The wetted circular area of the surface forms the working electrode (WE). The capillary contains reference (RE) and counter electrode (CE). The mouth diameter of the capillaries differed from 700 to 30 mm, depending on the particle size. Evaporation of solvent is avoided by a silicone rubber gasket formed at the mouth of the capillary. This gasket is slightly pressed against the sample so that the gasket defines the droplet volume exactly. The capillary
Fig. 2. REM micrograph of single nickel hydroxide particles on a gold sheet. This figure illustrates the size distribution of the material, for investigations of single particles the mean distance has to be increased.
T. Hamelmann, M.M. Lohrengel / Electrochimica Acta 47 (2001) 117–120
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capillaries. The mouth had to be mechanically polished until it showed an angle of 60° versus symmetry axis of the capillary.
3. Examples
3.1. Cementite particles
Fig. 4. Cyclovoltammogram (full line, left scale) of a 200 mm cementite particle on a gold sheet in 20% NaCl solution, inner capillary diameter 700 mm, sweep rate 50 mV/s, 25 °C and corresponding capacity at 1 kHz (dotted line, right scale).
is made of glass; the counter electrode is a small wire inside the capillary. The capillaries are fixed to a Plexiglass carrier which contains the micro reference electrode. It is equipped with special fittings for syringes (electrolyte support). The whole carrier is positioned above the sample by a XYZ-stage. Sample surface and droplet size are monitored by a video microscope. This cell design allows the application of all the common electrochemical techniques in a wide range: impedance spectroscopy (frequencies 1 mHzB fB 1 MHz), cyclovoltammetry (sweep rates up to 100 V/s), or current transients of potentiostatic steps (time 1 ms B tB 1000 s, current densities 100 nA/cm2 Bi B100 A/cm2) [3]. Usually, the surface is monitored by the video microscope under an angle of 45°. This is sufficient for many systems. During the investigation of single particles, however, we had to realize that an identification of very small particles (diameter of some mm) was difficult. Much better was a perpendicular view, which was normally blocked by the capillary. So we used inclined
Fig. 5. Cyclovoltammograms of a Ni(OH)2 particle, embedded in gold, diameter 9.9 mm, sweep rate 10 mV/s, 25 °C, in 6 M KOH. Shown are the 1st, 10th and 24th cycles.
Cementite (Fe3C) particles are typical inclusions in steel and influence the corrosion behavior. An investigation of the bulk material is hindered by the mechanical properties, e.g. brittleness, which disables the formation of wires or sheets. Furthermore, small particles can show a special structure and, hence, a different electrochemical behavior. Fig. 4 shows a cyclovoltammogram in 20% NaCl solution of a single cementite particle with a diameter of about 200 mm and the corresponding capacity at 1 kHz. The currents of the particle are more than one decade larger than the contribution of the gold sheet and, hence, dominate the spectra. A separation of the capacity data of Fe3C and Au is more critical. The capacity on pure Au, however, is almost constant up to 0.3 V. Therefore, the capacity maximum at 0.2 V must be attributed to the particle. Obviously, this indicates a reversible adsorption of some species as a first step of oxidation process, as the capacity decreases again with increasing current density.
3.2. Nickel hydroxide particles The electrochemical behavior of spherical Ni(OH)2 particles is important for the optimization of batteries [4]. In former investigations, large numbers of Ni(OH)2 particles were embedded by nickel [5]. Drawback of this method is the superposition of the Ni(OH)2 and the Ni spectra and the large number of simultaneously measured particles. In the experiments presented here, commercial particles of nickel hydroxide with a size from 5 to 15 mm were used. The electrochemical measurements were carried out in 6 M KOH. Potentials are given versus Hg/HgO electrode in 6 M KOH. Fig. 5 shows the 1st, 10th and 24th cycle in 6 M KOH. The redox charge increases with the number of cycles and indicates an activation process. The 1st cycle shows a run of the curve similar to a pure Au-CV. The typical peaks of oxidation and reduction become more and more pronounced. The contribution of the gold base is small (flat reduction peak at 0.16 V) and can be easily separated. Future experiments will show the influence of shape, structure, and size on the electrochemical properties.
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4. Summary Small electroactive particles are important materials in many technical processes. Examples for such systems are catalysts, batteries or local elements in corroding systems. The properties of these particles can depend on the composition, shape, and inner structure of the particles. Knowledge is important of the individual properties of single particles of different size or shape. A new method to investigate single micro particles with diameters in the range of 10 mm is presented. Small numbers of these particles are deposited on a gold sheet, fixed and contacted by a thin galvanized gold film and investigated by a capillary-based droplet cell. Cyclovoltammograms of different samples (cementite or Ni(OH)2 particles) are presented as examples. Some conditions, however, must be fulfilled to apply this technique. The conductivity of the particles must be low to avoid a deposition of gold on the surface of the sample during galvanization. In addition, the properties of the particles must not be influenced by this process.
.
Furthermore, the electrical signals of the particle must be separated from those of the gold base. This is true in many cases, because the double layer region of Au is large and the formation of oxide stops after some monolayers. This means the charge contribution is small. More critical is the separation of capacity, as this value is relative large on an oxide free Au electrode. References [1] H.-S. Kim, T. Itoh, M. Nishizawa, T. Abe, I. Uchida, The Electrochem. Soc., 99-92 (1999) 440. [2] T. Haisch, E. Mittemeijer, J.W. Schultze, Electrochim. Acta 47 (2001) 235. [3] (a) M.M. Lohrengel, Electrochim. Acta 42 (1997) 3265; (b) A.W. Hassel, M.M. Lohrengel, Electrochim. Acta 42 (1997) 3327; (c) M.M. Lohrengel, A. Moehring, M. Pilaski, Fresenius’ J. Anal. Chem. 367 (2000) 334. [4] (a) Y. Zhang, Z. Zhou, J. Yan, J. Power Sources 75 (1998) 283; (b) L. Bing, Y. Huatang, Z. Yunshi, Z. Zuoxiang, S. Deying, J. Power Sources 79 (1999) 277. [5] M. Keddam, S. Senyarich, H. Takenouti, J. Appl. Electrochem. 24 (1994) 1037.
Electrochemical investigations of single microparticles T. Hamelmann *, M.M. Lohrengel Institut fu¨r Physikalische Chemie und Elektrochemie, Heinrich-Heine-Uni6ersita¨t Du¨sseldorf, Uni6ersita¨tsstraße 1, 40225 Dusseldorf, Germany Received 14 December 2000; received in revised form 13 March 2001
Abstract Small electroactive particles are important materials in many technical processes. A new method to investigate single micro particles with diameters in the range of 10 mm is presented. Small numbers of these particles are fixed on a gold sheet and investigated by a capillary-based droplet cell. Cyclovoltammograms of different samples (cementite or Ni(OH)2 particles) are presented as examples. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microcell; Single particle; Nickel hydroxide
1. Introduction Well-defined electrodes (like wires or sheets) cannot be formed from all electrochemically relevant materials. Examples are powders which are used as an active material in batteries or brittle materials which forms inclusions. Macroscopic investigations of pressed, sintered or embedded particles are fundamentally possible but involve large numbers of particles. If their ionic or electronic conductivity is poor, potential or concentration gradients may develop. Furthermore, the individual properties may differ from particle to particle, due to size, shape, structure, crystallinity, or composition and conventional experiments yield mean data only. Therefore, it is necessary to investigate single particles with the common potentiostatic techniques like cyclovoltammograms, transients or impedance. In this paper, we present a convenient method to prepare sheets with small numbers of particles with a size of some micrometer and show how they can be addressed individually by a special electrochemical microcell. Other techniques to measure single particles, e.g. by contacting particles embedded in insulating materials with carbon fibers [1], are limited due to the high contact resistance. * Corresponding author. E-mail address: [email protected] (T. Hamelmann).
Two examples are presented. Nickel hydroxide particles (2–20 mm in diameter) are a base material for rechargeable Ni/Cd- or Ni/MH-oxide or LiC/LiNiO2batteries. Their activity depends on the phase composition, shape, and inner structure of the particles. Particles of Fe3C are typical inclusions in steel and influence the mechanical properties, electrochemical machining [2], and corrosion.
2. Experimental For these investigations, we had to solve two problems: preparation of isolated particles with a welldefined electric contact and the local electrochemical investigation.
2.1. Preparation of samples Our fundamental idea was to fix small numbers of particles on a metal sheet so that the mean distance between the particles is much larger than their diameter. This should enable us to address individual particles with the scanning droplet cell. As a base sheet we used gold because this metal has the largest double layer region of all noble metals (approximately −0.2 to 1.4 V vs. SHE at pH 0). Therefore, an influence of this base should be small. Other materials with a large double layer region are liquid or non-metallic and, therefore cannot be galvanically deposited (discussed
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 5 5 6 - 4
118
T. Hamelmann, M.M. Lohrengel / Electrochimica Acta 47 (2001) 117–120
Fig. 1. Schematic cross-section of single nickel hydroxide particles embedded in an electrodeposited gold layer.
later). Small amounts of particles were dispersed in pure water or organic solvents in an ultrasonic bath. The concentration of particles is very critical for the subsequent procedures. A defined small droplet of the suspension was pipetted on a gold sheet. After evaporation of the solvent, the particles adhered slightly to the gold sheet. The next step was to fix them irreversibly and to obtain a good electric contact with the gold sheet. This was done by galvanic embedding in gold, i.e. about 1.5 mm of gold was deposited (3 mA/cm2 for 500 s). This technique requires that no gold be deposited on the particles because the conductivity of the particles is small or if the formation of gold nuclei is hindered on the particle surface. Both may be true for the particles investigated here. This embedding procedure (Fig. 1) blocks parts of the particle surface typically in the range of 10% (assuming 1 mm of galvanic gold and 10 mm particles) but, on the other hand, guarantees a good contact (Fig. 2).
Fig. 3. Schematic presentation of single particles addressed with the droplet cell.
2.2. Electrochemical set-up A macroscopic investigation of a single particle on a gold sheet of e.g. 1 cm2 is impossible as side reactions of electroactive impurities from the electrolyte on the gold surface would suppress the response of the particle of some micrometers. Therefore, the electrochemical access had to be limited to a small region only around the particle. This becomes possible with the scanning droplet cell [3] (Fig. 3). An electrolyte droplet is dispensed and positioned on the surface by a capillary. The wetted circular area of the surface forms the working electrode (WE). The capillary contains reference (RE) and counter electrode (CE). The mouth diameter of the capillaries differed from 700 to 30 mm, depending on the particle size. Evaporation of solvent is avoided by a silicone rubber gasket formed at the mouth of the capillary. This gasket is slightly pressed against the sample so that the gasket defines the droplet volume exactly. The capillary
Fig. 2. REM micrograph of single nickel hydroxide particles on a gold sheet. This figure illustrates the size distribution of the material, for investigations of single particles the mean distance has to be increased.
T. Hamelmann, M.M. Lohrengel / Electrochimica Acta 47 (2001) 117–120
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capillaries. The mouth had to be mechanically polished until it showed an angle of 60° versus symmetry axis of the capillary.
3. Examples
3.1. Cementite particles
Fig. 4. Cyclovoltammogram (full line, left scale) of a 200 mm cementite particle on a gold sheet in 20% NaCl solution, inner capillary diameter 700 mm, sweep rate 50 mV/s, 25 °C and corresponding capacity at 1 kHz (dotted line, right scale).
is made of glass; the counter electrode is a small wire inside the capillary. The capillaries are fixed to a Plexiglass carrier which contains the micro reference electrode. It is equipped with special fittings for syringes (electrolyte support). The whole carrier is positioned above the sample by a XYZ-stage. Sample surface and droplet size are monitored by a video microscope. This cell design allows the application of all the common electrochemical techniques in a wide range: impedance spectroscopy (frequencies 1 mHzB fB 1 MHz), cyclovoltammetry (sweep rates up to 100 V/s), or current transients of potentiostatic steps (time 1 ms B tB 1000 s, current densities 100 nA/cm2 Bi B100 A/cm2) [3]. Usually, the surface is monitored by the video microscope under an angle of 45°. This is sufficient for many systems. During the investigation of single particles, however, we had to realize that an identification of very small particles (diameter of some mm) was difficult. Much better was a perpendicular view, which was normally blocked by the capillary. So we used inclined
Fig. 5. Cyclovoltammograms of a Ni(OH)2 particle, embedded in gold, diameter 9.9 mm, sweep rate 10 mV/s, 25 °C, in 6 M KOH. Shown are the 1st, 10th and 24th cycles.
Cementite (Fe3C) particles are typical inclusions in steel and influence the corrosion behavior. An investigation of the bulk material is hindered by the mechanical properties, e.g. brittleness, which disables the formation of wires or sheets. Furthermore, small particles can show a special structure and, hence, a different electrochemical behavior. Fig. 4 shows a cyclovoltammogram in 20% NaCl solution of a single cementite particle with a diameter of about 200 mm and the corresponding capacity at 1 kHz. The currents of the particle are more than one decade larger than the contribution of the gold sheet and, hence, dominate the spectra. A separation of the capacity data of Fe3C and Au is more critical. The capacity on pure Au, however, is almost constant up to 0.3 V. Therefore, the capacity maximum at 0.2 V must be attributed to the particle. Obviously, this indicates a reversible adsorption of some species as a first step of oxidation process, as the capacity decreases again with increasing current density.
3.2. Nickel hydroxide particles The electrochemical behavior of spherical Ni(OH)2 particles is important for the optimization of batteries [4]. In former investigations, large numbers of Ni(OH)2 particles were embedded by nickel [5]. Drawback of this method is the superposition of the Ni(OH)2 and the Ni spectra and the large number of simultaneously measured particles. In the experiments presented here, commercial particles of nickel hydroxide with a size from 5 to 15 mm were used. The electrochemical measurements were carried out in 6 M KOH. Potentials are given versus Hg/HgO electrode in 6 M KOH. Fig. 5 shows the 1st, 10th and 24th cycle in 6 M KOH. The redox charge increases with the number of cycles and indicates an activation process. The 1st cycle shows a run of the curve similar to a pure Au-CV. The typical peaks of oxidation and reduction become more and more pronounced. The contribution of the gold base is small (flat reduction peak at 0.16 V) and can be easily separated. Future experiments will show the influence of shape, structure, and size on the electrochemical properties.
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T. Hamelmann, M.M. Lohrengel / Electrochimica Acta 47 (2001) 117–120
4. Summary Small electroactive particles are important materials in many technical processes. Examples for such systems are catalysts, batteries or local elements in corroding systems. The properties of these particles can depend on the composition, shape, and inner structure of the particles. Knowledge is important of the individual properties of single particles of different size or shape. A new method to investigate single micro particles with diameters in the range of 10 mm is presented. Small numbers of these particles are deposited on a gold sheet, fixed and contacted by a thin galvanized gold film and investigated by a capillary-based droplet cell. Cyclovoltammograms of different samples (cementite or Ni(OH)2 particles) are presented as examples. Some conditions, however, must be fulfilled to apply this technique. The conductivity of the particles must be low to avoid a deposition of gold on the surface of the sample during galvanization. In addition, the properties of the particles must not be influenced by this process.
.
Furthermore, the electrical signals of the particle must be separated from those of the gold base. This is true in many cases, because the double layer region of Au is large and the formation of oxide stops after some monolayers. This means the charge contribution is small. More critical is the separation of capacity, as this value is relative large on an oxide free Au electrode. References [1] H.-S. Kim, T. Itoh, M. Nishizawa, T. Abe, I. Uchida, The Electrochem. Soc., 99-92 (1999) 440. [2] T. Haisch, E. Mittemeijer, J.W. Schultze, Electrochim. Acta 47 (2001) 235. [3] (a) M.M. Lohrengel, Electrochim. Acta 42 (1997) 3265; (b) A.W. Hassel, M.M. Lohrengel, Electrochim. Acta 42 (1997) 3327; (c) M.M. Lohrengel, A. Moehring, M. Pilaski, Fresenius’ J. Anal. Chem. 367 (2000) 334. [4] (a) Y. Zhang, Z. Zhou, J. Yan, J. Power Sources 75 (1998) 283; (b) L. Bing, Y. Huatang, Z. Yunshi, Z. Zuoxiang, S. Deying, J. Power Sources 79 (1999) 277. [5] M. Keddam, S. Senyarich, H. Takenouti, J. Appl. Electrochem. 24 (1994) 1037.