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The electrodeposition of highly reflective lead dioxide coatings
Electrochemistry Communications 11 (2009) 1301–1304
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
The electrodeposition of highly reflective lead dioxide coatings C.T.J. Low a,*, D. Pletcher b, F.C. Walsh a a
Electrochemical Engineering Laboratory, Energy Technology Research Group, School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom Electrochemistry and Surface Science Group, School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
b
a r t i c l e
i n f o
Article history: Received 28 March 2009 Received in revised form 23 April 2009 Accepted 28 April 2009 Available online 6 May 2009 Keywords: Electrodeposition Reflective Lead dioxide Methanesulfonic acid
a b s t r a c t In the presence of a suitable surfactant, such as hexadecyltrimethylammonium chloride or bromide, highly reflective and hard lead dioxide coatings with a black appearance can be electrodeposited from methanesulfonic acid media at room temperature (295 K). The reflective PbO2 coatings are compact, adherent to the (vitreous carbon or carbon-polymer) substrate and can be formed at current densities of 10 to 100 mA cm 2 at a thickness up to several hundred microns. The coatings were characterised by measurement of surface optical reflectance, surface roughness, surface microstructure, phase composition and crystallite size. The reflective PbO2 films were found to mainly consist of the alpha (orthorhombic) phase with feather-like and orientated microstructures. The crystallite size and surface roughness were in the order of tens of nanometres and their optical reflectance was several orders of magnitude higher than matte coatings produced in the absence of additives. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Recent years have seen extensive studies of the anodic deposition of lead dioxide onto inert substrates, as seen in [1–3] and references therein. Such coatings are relatively low in cost and can provide stable and robust structures for anodic oxidation at very positive potentials. Most recently, the focus of development has been to identify preparations of lead dioxide with improved characteristics e.g. higher catalytic activity and enhanced stability. In Southampton, interest has focused on deposition from methanesulfonic acid media [4–10] because lead(II) is highly soluble and this acid is non-oxidising, relatively non-corrosive and biodegradable [11]. In this communication, we report the anodic deposition of lead dioxide deposits on carbon substrates with unusual characteristics. The deposits are black, hard and highly reflecting. 2. Experimental details The electrolytes contained 0.5–1.5 mol dm 3 lead(II) methanesulfonate, 0.2–1.5 mol dm 3 methanesulfonic acid. The additives used were hexadecyltrimethylammonium chloride, bromide, hydroxide and 0.1–1 mmol dm 3 p-toluenesulfonate. All chemicals were analytical reagent grade from Sigma Aldrich UK. All electrochemical measurements were made with an EcoChemie Autolab (PGSTAT20) computer controlled potentiostat using
* Corresponding author. Tel.: +44 (0)23 8059 7052; fax: +44 (0)23 8059 7051. E-mail address: [email protected] (C.T.J. Low).
the General Purpose, Electrochemical Software (GPES) Version 4.5. Cyclic voltammetry was performed at a static glassy carbon disc electrode (area, 0.126 cm2) in a three-electrode, two-compartment glass cell. Constant current deposition was carried out in an undivided parallel plate, ‘beaker’ cell at current densities from 10 to 100 mA cm 2. The deposition time ranged from 10 minutes to 2 h and the temperature of the electrolyte was controlled at 283–333 K. The electrolyte volume was approximately 70 cm3. A magnetic stirrer (PTFE-coated steel cylindrical stirrer bar, 4.5 cm length and 0.8 cm diameter), c.a. 300 rpm, was used to generate flow in the solution. The working electrode (1.0 cm width and 4.0 cm height) was a carbon-polymer composite (Type: BMC940) from Entegris GmbH, Germany and the sides and back were covered with insulating material. Nickel plate (4 cm 1 cm) was the counter electrode. The interelectrode separation was 1.0 cm. Before each experiment, all electrodes were mechanically roughened using 1200 and 4000 grade SiC paper then ultrasonically cleaned in detergent and distilled water. Surface roughness was measured using an atomic force microscope from Asylum Research MFP-3DTM AFM. Silicon tips were used and the measurements were carried out using AC mode imaging in the open atmosphere. X-ray diffraction patterns were measured using a Siemens D5000 diffractometer, 2h from 20 to 90° using Cu Ka, k = 0.154056 nm. Optical reflectance index was measured using an Avantes Avanspec-2048FT (Anglia Instruments Ltd.) device at wavelengths 460–910 nm. Microstructural characterisations were carried out using a scanning electron microscopy and energy dispersive X-ray spectroscopy (XL30ESEM, Philips), a 15 kV accelerating voltage and 5 minutes collection time.
1388-2481/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.04.032
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C.T.J. Low et al. / Electrochemistry Communications 11 (2009) 1301–1304
3. Results and discussion 3.1. Cyclic voltammetry of Pb2+/PbO2 Cyclic voltammograms recorded for solutions of 0.5 mol dm 3 Pb(II) in 0.2 mol dm 3 methanesulfonic acid were essentially identical to those reported earlier [4]. Large anodic current densities for the deposition of lead dioxide were observed at potentials more positive than +1.5 V vs. SCE and less positive than the oxygen evolution reaction. In addition, a symmetrical cathodic peak for the reduction of the lead dioxide at c.a. +1.0 V vs. SCE was seen on the reverse scan. The addition of 1 mmol dm 3 of the hexadecyltrimethylammonium salts resulted in no significant difference to the voltammograms or the current density associated with the peaks associated with the PbO2/Pb2+ couple. 3.2. Optical reflectance Two PbO2 layers were deposited from a solution containing 0.5 mol dm 3 Pb(II) +0.2 mol dm 3 methanesulfonic acid using a current density of 20 mA cm 2 for 600 s at 295 K; for one deposition, no additive deposit was present and for the other, the solution also contained 0.8 mmol dm 3 hexadecyltrimethylammonium chloride. Fig. 1 shows the optical reflectance spectra for the two deposits as well as the carbon-polymer composite without a coating (set at 100%). Without additive in the electrolyte, the deposit had a matte and a slightly rough appearance; the optical reflectivity of the surface was poor compared to the substrate at all wavelengths. With addition of 0.8 mmol dm 3 to the electrolyte, the deposit had a black appearance that reflected light strongly (having a mirror finish). It can be seen from curve 1(a), that at all wavelengths, the reflectivity was high compared to the substrate carbon-polymer composite. It is also more than a factor of ten times more reflective than the matte deposit, curve 1(c). The reflectivity was particularly high at lower wavelengths. Fig. 2 reports the influence of current density and temperature on the reflectivity of deposits from the same solution containing 0.8 mmol dm 3 hexadecyltrimethylammonium chloride. For Fig. 2A, the temperature was 295 K and the deposition was carried
3.3. Surface microstructure PbO2 coatings could be readily deposited to achieve film thickness ranging from a fraction of a micron to several millimetres. The formation of cracks and lifting of the PbO2 deposit from the substrate determines the maximum film thickness but the maximum thickness depends on the conditions used for the deposition. Fig. 3 shows SEM images of the surface microstructures of PbO2 deposited from an electrolyte containing (A) no additive and (B) with the addition of 0.8 mmol dm 3 hexadecyltrimethylammonium chloride. The coatings were deposited at 20 mA cm 2 for 10 min at 295 K. In the absence of electrolyte additive, the coating had a matte-grey appearance and on the micron scale it consists of randomly orientated, largely ‘square’ crystallites. In the presence of the additive, the surface structure is quite different and consisted of small ‘feather’ like crystallites. Again, the addition of hexadecyltrimethylammonium bromide (but not the p-toluenesulfonate or hydroxide) led to the change is microstructure. It appears that the halide ion is critical in determining the surface structure, maybe by influencing the nucleation of the PbO2 phase via surface adsorption. 3.4. Surface roughness
600
Lead dioxide films were deposited over a range of controlled conditions and the surface roughness was determined by AFM. In Table 1, the surface roughness, as the Root Mean Square (RMS) value, is reported. The smoothest surfaces required the presence of a halide ion and were best achieved at lower temperature and higher current density and, at least initially, the surface becomes less rough as the layer thickened.
500
% optical reflectance
out for 3600 s. The trend is to a higher reflectivity with increase in current density and the appearance became more intensely black at higher current density. The thickness of the deposit formed at 100 mA cm 2 is approached 0.5 mm. For Fig. 2B, deposition took place at a current density of 20 mA cm 2 for 1 h. Clearly, temperature had a strong influence on the deposit characteristics. While the deposit had a high reflectivity at 283 K, the surface resulting from deposition at 333 K was grey and matte with a very poor reflectivity. An increase in reflectivity could also be achieved using hexadecyltrimethylammonium bromide as the additive. Concentrations of the chloride and bromide in the range 0.1–1.0 mmol dm 3 were suitable to give a highly reflecting surface. The addition of hexadecyltrimethylammonium p-toluenesulfonate and hydroxide to the electrolyte did not lead to an increase in reflectivity. It appears that the halide ion has a key role in increasing the reflectivity; adsorption of halide on the lead dioxide surface or complexation of the Pb2+ are likely reasons.
400
300
3.5. X-ray diffraction patterns (a)
200
(b) 100 (c) 0 460 510 560 610 660 710 760 810 860 910
Wavelength / nm Fig. 1. Optical reflectance of (a) reflective PbO2 coating, (b) carbon polymer substrate and (c) matte PbO2 coating. The matte coating was deposited in the absence of an additive while the reflective coating was deposited in the presence of 0.8 mmol dm 3 C17H33(CH3)3NCl. Both coatings were deposited at 20 mA cm 2 for 600 s from 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H at 295 K.
PbO2 is known to have two common crystallographic structures, namely the alpha- (orthorhombic) and beta- (tetragonal) forms. Fig. 4 shows the x-ray diffraction patterns for (A) matte and (B) reflective PbO2 coatings. Both the matte and reflective coatings had an a-PbO2 crystalline structure [12] but showed distinctly different x-ray diffraction patterns. The reflective coating had a strong preferential orientation along the (0 2 0) and (1 2 3)/(0 4 0) plane. For the matte coating, these peaks were not observed but contained various peaks having a strong intensity along the (0 0 2) and (0 2 3) planes. These findings are consistent with different layer structures. Also reported in the table is the mean grain size, which was estimated from the width of the strongest diffraction peak using the Debye–Sherrer equation [13].
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C.T.J. Low et al. / Electrochemistry Communications 11 (2009) 1301–1304
A
600
B
500
% optical reflectance
% optical reflectance
500
400
600
(c)
300 (b) 200
400
300 (a) 200
(a)
(b)
100
100
0 460 510 560 610 660 710 760 810 860 910
(c) 0 460 510 560 610 660 710 760 810 860 910
Wavelength (nm)
Wavelength (nm)
Fig. 2. Effects of operating parameters on the optical reflectance of PbO2 coatings. (A) Applied current density: (a) 10 mA cm 2, (b) 50 mA cm 2 and (c) 100 mA cm 2. (B) Temperature: (a) 283 K, (b) 303 K and (c) 333 K .The coatings were deposited from 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H + 0.8 mmol dm 3 C17H33(CH3)3NCl for 3600 s. For figure (A) the temperature was 295 K and figure (B) the current density was 20 mA cm 2.
Fig. 3. Surface microstructure of PbO2 coatings. (A) Matte PbO2 coating deposited from an electrolyte containing no additive at 20 mA cm 2 for 600 s at 295 K and (B) reflective PbO2 coating deposited from an electrolyte containing 0.8 mmol dm 3 C17H33(CH3)3NCl. 10 mA cm 2 for 1 h at 295 K. Electrolyte: 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H.
Table 1 Properties of PbO2 deposits onto a carbon/polymer composite anode from 0.5 mol dm 3 Pb(CH3SO3)2 + 0.2 mol dm 3 CH3SO3H. Unless indicated, the current density was 20 mA cm 2, deposition time 3600 s, temperature 295 K and the solution contained 0.5 mmol dm 3 hexadecyltrimethylammonium chloride. Operating parameters
Surface reflectivity
Electrolyte temperature/K 283 High 303 Poor 333 Poor Hexadecyltrimethylammonium additive None Poor Chloride High Bromide High Hydroxide Poor p-Toluenesulfonate Poor Current density/mA cm 2 10 Poor 50 High 100 High Deposition time/s 600 Poor 1800 High 7200 High
RMS surface roughness/nm
Phase composition
Crystallite size/nm
11 54 370
a a,b b
11 16 22
121 12 12 133 123
a,b a a a a
12 12 12 18 16
46 16 17
a a a
11 12 12
118 30 45
a a a
11 11 13
C.T.J. Low et al. / Electrochemistry Communications 11 (2009) 1301–1304
1600
B
α (002)
A
3000
α (020)
1304
1400
2500 1200
Intensity
α (023)
0 20
1500
1000
500
α (002)
α (332) α (323) α (134)
α (004) α (223)
α (113)
α (130)
α (022)
α (112)
α (200)
200
α (111)
400
α (021)
600
α (241)
Intensity
800
α (123)/(040)
2000
1000
0 40
60
80
20
40
60
80
2θ
2θ
Fig. 4. X-ray diffraction patterns of (a) matte and (b) reflective lead dioxide coating. The coatings were deposited at 20 mA cm 2 for 10 min at 295 K from electrolytes containing 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H. (A) No electrolyte additive (B) the electrolyte contained 0.5 mmol dm 3 C17H33(CH3)3NCl.
It was found that the phase composition did not change with the additive or its concentration, current density or thickness of the deposit. On the other hand, with increasing temperature, the composition changed to a mixture of a-PbO2 and b-PbO2 and by 333 K the layer was pure b-PbO2 [10]. There is also a trend for the crystallite size to increase with the change from a highly reflecting to matte surfaces. 4. Conclusions It has been shown that very highly reflective lead dioxide coatings with a black appearance can be produced from methanesulfonic acid electrolytes. Indeed, while the high reflectivity depends on the presence of a halide ion and low temperature, they could be deposited from a range of electrolyte conditions: such surfaces have been produced with a range of thicknesses from solutions containing 0.5–1.5 mol dm 3 lead(II) methanesulfonate, 0.2– 1.5 mol dm 3 methanesulfonic acid and 0.1–1 mmol dm 3 hexadecyltrimethylammonium chloride or bromide. As is to be expected, there is an excellent correlation between the reflectivity and the size of the surface features as monitored by SEM and AFM. The
highly reflective coatings always had a pure a-PbO2 phase composition. References [1] D. Devilliers, M.T. Dinh Thi, E. Mahé, V. Dauriac, N. Lequeux, J. Electroanal. Chem. 573 (2004) 227. [2] Y. Mohd, D. Pletcher, J. Electrochem. Soc. 152 (2005) D97. [3] A.B. Velichenko, R. Amadelli, E.V. Gruzdeva, T.V. Luk’yanenko, F.I. Davilov, J. Power Sources 191 (2009) 103. [4] A. Hazza, D. Pletcher, R. Wills, Phys. Chem. Chem. Phys. 6 (2004) 1773. [5] D. Pletcher, R. Wills, Phys. Chem. Chem. Phys. 6 (2004) 1779. [6] D. Pletcher, R. Wills, J. Power Sources 149 (2005) 96. [7] A. Hazza, D. Pletcher, R. Wills, J. Power Sources 149 (2005) 103. [8] D. Pletcher, H. Zhou, G. Kear, C.T.J. Low, F.C. Walsh, R. Wills, J. Power Sources 180 (2008) 621. [9] D. Pletcher, H. Zhou, G. Kear, C.T.J. Low, F.C. Walsh, R. Wills, J. Power Sources 180 (2008) 630. [10] X. Li, D. Pletcher, F.C. Walsh, Electrochim Acta 54 (2009) 4688. [11] M.D. Gernon, M. Wu, T. Buszta, P. Janney, Green Chem. (1999) 127. [12] International Centre for Diffraction Data Power Diffraction File, ICDD, Philadelphia, PA, Card No. 72-2440 for a-PbO2 and Card No. 76-0564 for bPbO2, 2001. [13] A.L. Patterson, Phys. Rev. 56 (1939) 978.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
The electrodeposition of highly reflective lead dioxide coatings C.T.J. Low a,*, D. Pletcher b, F.C. Walsh a a
Electrochemical Engineering Laboratory, Energy Technology Research Group, School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom Electrochemistry and Surface Science Group, School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
b
a r t i c l e
i n f o
Article history: Received 28 March 2009 Received in revised form 23 April 2009 Accepted 28 April 2009 Available online 6 May 2009 Keywords: Electrodeposition Reflective Lead dioxide Methanesulfonic acid
a b s t r a c t In the presence of a suitable surfactant, such as hexadecyltrimethylammonium chloride or bromide, highly reflective and hard lead dioxide coatings with a black appearance can be electrodeposited from methanesulfonic acid media at room temperature (295 K). The reflective PbO2 coatings are compact, adherent to the (vitreous carbon or carbon-polymer) substrate and can be formed at current densities of 10 to 100 mA cm 2 at a thickness up to several hundred microns. The coatings were characterised by measurement of surface optical reflectance, surface roughness, surface microstructure, phase composition and crystallite size. The reflective PbO2 films were found to mainly consist of the alpha (orthorhombic) phase with feather-like and orientated microstructures. The crystallite size and surface roughness were in the order of tens of nanometres and their optical reflectance was several orders of magnitude higher than matte coatings produced in the absence of additives. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Recent years have seen extensive studies of the anodic deposition of lead dioxide onto inert substrates, as seen in [1–3] and references therein. Such coatings are relatively low in cost and can provide stable and robust structures for anodic oxidation at very positive potentials. Most recently, the focus of development has been to identify preparations of lead dioxide with improved characteristics e.g. higher catalytic activity and enhanced stability. In Southampton, interest has focused on deposition from methanesulfonic acid media [4–10] because lead(II) is highly soluble and this acid is non-oxidising, relatively non-corrosive and biodegradable [11]. In this communication, we report the anodic deposition of lead dioxide deposits on carbon substrates with unusual characteristics. The deposits are black, hard and highly reflecting. 2. Experimental details The electrolytes contained 0.5–1.5 mol dm 3 lead(II) methanesulfonate, 0.2–1.5 mol dm 3 methanesulfonic acid. The additives used were hexadecyltrimethylammonium chloride, bromide, hydroxide and 0.1–1 mmol dm 3 p-toluenesulfonate. All chemicals were analytical reagent grade from Sigma Aldrich UK. All electrochemical measurements were made with an EcoChemie Autolab (PGSTAT20) computer controlled potentiostat using
* Corresponding author. Tel.: +44 (0)23 8059 7052; fax: +44 (0)23 8059 7051. E-mail address: [email protected] (C.T.J. Low).
the General Purpose, Electrochemical Software (GPES) Version 4.5. Cyclic voltammetry was performed at a static glassy carbon disc electrode (area, 0.126 cm2) in a three-electrode, two-compartment glass cell. Constant current deposition was carried out in an undivided parallel plate, ‘beaker’ cell at current densities from 10 to 100 mA cm 2. The deposition time ranged from 10 minutes to 2 h and the temperature of the electrolyte was controlled at 283–333 K. The electrolyte volume was approximately 70 cm3. A magnetic stirrer (PTFE-coated steel cylindrical stirrer bar, 4.5 cm length and 0.8 cm diameter), c.a. 300 rpm, was used to generate flow in the solution. The working electrode (1.0 cm width and 4.0 cm height) was a carbon-polymer composite (Type: BMC940) from Entegris GmbH, Germany and the sides and back were covered with insulating material. Nickel plate (4 cm 1 cm) was the counter electrode. The interelectrode separation was 1.0 cm. Before each experiment, all electrodes were mechanically roughened using 1200 and 4000 grade SiC paper then ultrasonically cleaned in detergent and distilled water. Surface roughness was measured using an atomic force microscope from Asylum Research MFP-3DTM AFM. Silicon tips were used and the measurements were carried out using AC mode imaging in the open atmosphere. X-ray diffraction patterns were measured using a Siemens D5000 diffractometer, 2h from 20 to 90° using Cu Ka, k = 0.154056 nm. Optical reflectance index was measured using an Avantes Avanspec-2048FT (Anglia Instruments Ltd.) device at wavelengths 460–910 nm. Microstructural characterisations were carried out using a scanning electron microscopy and energy dispersive X-ray spectroscopy (XL30ESEM, Philips), a 15 kV accelerating voltage and 5 minutes collection time.
1388-2481/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.04.032
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C.T.J. Low et al. / Electrochemistry Communications 11 (2009) 1301–1304
3. Results and discussion 3.1. Cyclic voltammetry of Pb2+/PbO2 Cyclic voltammograms recorded for solutions of 0.5 mol dm 3 Pb(II) in 0.2 mol dm 3 methanesulfonic acid were essentially identical to those reported earlier [4]. Large anodic current densities for the deposition of lead dioxide were observed at potentials more positive than +1.5 V vs. SCE and less positive than the oxygen evolution reaction. In addition, a symmetrical cathodic peak for the reduction of the lead dioxide at c.a. +1.0 V vs. SCE was seen on the reverse scan. The addition of 1 mmol dm 3 of the hexadecyltrimethylammonium salts resulted in no significant difference to the voltammograms or the current density associated with the peaks associated with the PbO2/Pb2+ couple. 3.2. Optical reflectance Two PbO2 layers were deposited from a solution containing 0.5 mol dm 3 Pb(II) +0.2 mol dm 3 methanesulfonic acid using a current density of 20 mA cm 2 for 600 s at 295 K; for one deposition, no additive deposit was present and for the other, the solution also contained 0.8 mmol dm 3 hexadecyltrimethylammonium chloride. Fig. 1 shows the optical reflectance spectra for the two deposits as well as the carbon-polymer composite without a coating (set at 100%). Without additive in the electrolyte, the deposit had a matte and a slightly rough appearance; the optical reflectivity of the surface was poor compared to the substrate at all wavelengths. With addition of 0.8 mmol dm 3 to the electrolyte, the deposit had a black appearance that reflected light strongly (having a mirror finish). It can be seen from curve 1(a), that at all wavelengths, the reflectivity was high compared to the substrate carbon-polymer composite. It is also more than a factor of ten times more reflective than the matte deposit, curve 1(c). The reflectivity was particularly high at lower wavelengths. Fig. 2 reports the influence of current density and temperature on the reflectivity of deposits from the same solution containing 0.8 mmol dm 3 hexadecyltrimethylammonium chloride. For Fig. 2A, the temperature was 295 K and the deposition was carried
3.3. Surface microstructure PbO2 coatings could be readily deposited to achieve film thickness ranging from a fraction of a micron to several millimetres. The formation of cracks and lifting of the PbO2 deposit from the substrate determines the maximum film thickness but the maximum thickness depends on the conditions used for the deposition. Fig. 3 shows SEM images of the surface microstructures of PbO2 deposited from an electrolyte containing (A) no additive and (B) with the addition of 0.8 mmol dm 3 hexadecyltrimethylammonium chloride. The coatings were deposited at 20 mA cm 2 for 10 min at 295 K. In the absence of electrolyte additive, the coating had a matte-grey appearance and on the micron scale it consists of randomly orientated, largely ‘square’ crystallites. In the presence of the additive, the surface structure is quite different and consisted of small ‘feather’ like crystallites. Again, the addition of hexadecyltrimethylammonium bromide (but not the p-toluenesulfonate or hydroxide) led to the change is microstructure. It appears that the halide ion is critical in determining the surface structure, maybe by influencing the nucleation of the PbO2 phase via surface adsorption. 3.4. Surface roughness
600
Lead dioxide films were deposited over a range of controlled conditions and the surface roughness was determined by AFM. In Table 1, the surface roughness, as the Root Mean Square (RMS) value, is reported. The smoothest surfaces required the presence of a halide ion and were best achieved at lower temperature and higher current density and, at least initially, the surface becomes less rough as the layer thickened.
500
% optical reflectance
out for 3600 s. The trend is to a higher reflectivity with increase in current density and the appearance became more intensely black at higher current density. The thickness of the deposit formed at 100 mA cm 2 is approached 0.5 mm. For Fig. 2B, deposition took place at a current density of 20 mA cm 2 for 1 h. Clearly, temperature had a strong influence on the deposit characteristics. While the deposit had a high reflectivity at 283 K, the surface resulting from deposition at 333 K was grey and matte with a very poor reflectivity. An increase in reflectivity could also be achieved using hexadecyltrimethylammonium bromide as the additive. Concentrations of the chloride and bromide in the range 0.1–1.0 mmol dm 3 were suitable to give a highly reflecting surface. The addition of hexadecyltrimethylammonium p-toluenesulfonate and hydroxide to the electrolyte did not lead to an increase in reflectivity. It appears that the halide ion has a key role in increasing the reflectivity; adsorption of halide on the lead dioxide surface or complexation of the Pb2+ are likely reasons.
400
300
3.5. X-ray diffraction patterns (a)
200
(b) 100 (c) 0 460 510 560 610 660 710 760 810 860 910
Wavelength / nm Fig. 1. Optical reflectance of (a) reflective PbO2 coating, (b) carbon polymer substrate and (c) matte PbO2 coating. The matte coating was deposited in the absence of an additive while the reflective coating was deposited in the presence of 0.8 mmol dm 3 C17H33(CH3)3NCl. Both coatings were deposited at 20 mA cm 2 for 600 s from 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H at 295 K.
PbO2 is known to have two common crystallographic structures, namely the alpha- (orthorhombic) and beta- (tetragonal) forms. Fig. 4 shows the x-ray diffraction patterns for (A) matte and (B) reflective PbO2 coatings. Both the matte and reflective coatings had an a-PbO2 crystalline structure [12] but showed distinctly different x-ray diffraction patterns. The reflective coating had a strong preferential orientation along the (0 2 0) and (1 2 3)/(0 4 0) plane. For the matte coating, these peaks were not observed but contained various peaks having a strong intensity along the (0 0 2) and (0 2 3) planes. These findings are consistent with different layer structures. Also reported in the table is the mean grain size, which was estimated from the width of the strongest diffraction peak using the Debye–Sherrer equation [13].
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C.T.J. Low et al. / Electrochemistry Communications 11 (2009) 1301–1304
A
600
B
500
% optical reflectance
% optical reflectance
500
400
600
(c)
300 (b) 200
400
300 (a) 200
(a)
(b)
100
100
0 460 510 560 610 660 710 760 810 860 910
(c) 0 460 510 560 610 660 710 760 810 860 910
Wavelength (nm)
Wavelength (nm)
Fig. 2. Effects of operating parameters on the optical reflectance of PbO2 coatings. (A) Applied current density: (a) 10 mA cm 2, (b) 50 mA cm 2 and (c) 100 mA cm 2. (B) Temperature: (a) 283 K, (b) 303 K and (c) 333 K .The coatings were deposited from 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H + 0.8 mmol dm 3 C17H33(CH3)3NCl for 3600 s. For figure (A) the temperature was 295 K and figure (B) the current density was 20 mA cm 2.
Fig. 3. Surface microstructure of PbO2 coatings. (A) Matte PbO2 coating deposited from an electrolyte containing no additive at 20 mA cm 2 for 600 s at 295 K and (B) reflective PbO2 coating deposited from an electrolyte containing 0.8 mmol dm 3 C17H33(CH3)3NCl. 10 mA cm 2 for 1 h at 295 K. Electrolyte: 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H.
Table 1 Properties of PbO2 deposits onto a carbon/polymer composite anode from 0.5 mol dm 3 Pb(CH3SO3)2 + 0.2 mol dm 3 CH3SO3H. Unless indicated, the current density was 20 mA cm 2, deposition time 3600 s, temperature 295 K and the solution contained 0.5 mmol dm 3 hexadecyltrimethylammonium chloride. Operating parameters
Surface reflectivity
Electrolyte temperature/K 283 High 303 Poor 333 Poor Hexadecyltrimethylammonium additive None Poor Chloride High Bromide High Hydroxide Poor p-Toluenesulfonate Poor Current density/mA cm 2 10 Poor 50 High 100 High Deposition time/s 600 Poor 1800 High 7200 High
RMS surface roughness/nm
Phase composition
Crystallite size/nm
11 54 370
a a,b b
11 16 22
121 12 12 133 123
a,b a a a a
12 12 12 18 16
46 16 17
a a a
11 12 12
118 30 45
a a a
11 11 13
C.T.J. Low et al. / Electrochemistry Communications 11 (2009) 1301–1304
1600
B
α (002)
A
3000
α (020)
1304
1400
2500 1200
Intensity
α (023)
0 20
1500
1000
500
α (002)
α (332) α (323) α (134)
α (004) α (223)
α (113)
α (130)
α (022)
α (112)
α (200)
200
α (111)
400
α (021)
600
α (241)
Intensity
800
α (123)/(040)
2000
1000
0 40
60
80
20
40
60
80
2θ
2θ
Fig. 4. X-ray diffraction patterns of (a) matte and (b) reflective lead dioxide coating. The coatings were deposited at 20 mA cm 2 for 10 min at 295 K from electrolytes containing 0.5 mol dm 3 Pb(CH3SO3)2 and 0.2 mol dm 3 CH3SO3H. (A) No electrolyte additive (B) the electrolyte contained 0.5 mmol dm 3 C17H33(CH3)3NCl.
It was found that the phase composition did not change with the additive or its concentration, current density or thickness of the deposit. On the other hand, with increasing temperature, the composition changed to a mixture of a-PbO2 and b-PbO2 and by 333 K the layer was pure b-PbO2 [10]. There is also a trend for the crystallite size to increase with the change from a highly reflecting to matte surfaces. 4. Conclusions It has been shown that very highly reflective lead dioxide coatings with a black appearance can be produced from methanesulfonic acid electrolytes. Indeed, while the high reflectivity depends on the presence of a halide ion and low temperature, they could be deposited from a range of electrolyte conditions: such surfaces have been produced with a range of thicknesses from solutions containing 0.5–1.5 mol dm 3 lead(II) methanesulfonate, 0.2– 1.5 mol dm 3 methanesulfonic acid and 0.1–1 mmol dm 3 hexadecyltrimethylammonium chloride or bromide. As is to be expected, there is an excellent correlation between the reflectivity and the size of the surface features as monitored by SEM and AFM. The
highly reflective coatings always had a pure a-PbO2 phase composition. References [1] D. Devilliers, M.T. Dinh Thi, E. Mahé, V. Dauriac, N. Lequeux, J. Electroanal. Chem. 573 (2004) 227. [2] Y. Mohd, D. Pletcher, J. Electrochem. Soc. 152 (2005) D97. [3] A.B. Velichenko, R. Amadelli, E.V. Gruzdeva, T.V. Luk’yanenko, F.I. Davilov, J. Power Sources 191 (2009) 103. [4] A. Hazza, D. Pletcher, R. Wills, Phys. Chem. Chem. Phys. 6 (2004) 1773. [5] D. Pletcher, R. Wills, Phys. Chem. Chem. Phys. 6 (2004) 1779. [6] D. Pletcher, R. Wills, J. Power Sources 149 (2005) 96. [7] A. Hazza, D. Pletcher, R. Wills, J. Power Sources 149 (2005) 103. [8] D. Pletcher, H. Zhou, G. Kear, C.T.J. Low, F.C. Walsh, R. Wills, J. Power Sources 180 (2008) 621. [9] D. Pletcher, H. Zhou, G. Kear, C.T.J. Low, F.C. Walsh, R. Wills, J. Power Sources 180 (2008) 630. [10] X. Li, D. Pletcher, F.C. Walsh, Electrochim Acta 54 (2009) 4688. [11] M.D. Gernon, M. Wu, T. Buszta, P. Janney, Green Chem. (1999) 127. [12] International Centre for Diffraction Data Power Diffraction File, ICDD, Philadelphia, PA, Card No. 72-2440 for a-PbO2 and Card No. 76-0564 for bPbO2, 2001. [13] A.L. Patterson, Phys. Rev. 56 (1939) 978.