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Thermal preparation of thin platinum coatings and their electrochemical and atomic force microscopic characterization
Applied Surface Science 156 Ž2000. 135–142 www.elsevier.nlrlocaterapsusc
Thermal preparation of thin platinum coatings and their electrochemical and atomic force microscopic characterization K. Tammeveski a , T. Tenno a , J. Niinisto¨ b,c , T. Leitner b, G. Friedbacher b, L. Niinisto¨ c,) a
Institute of Physical Chemistry, UniÕersity of Tartu, Jakobi 2, 51014 Tartu, Estonia Institute of Analytical Chemistry, Vienna UniÕersity of Technology, Getreidemarkt 9 r 151 A-1060 Vienna, Austria Laboratory of Inorganic and Analytical Chemistry, Helsinki UniÕersity of Technology, P.O. Box 6100, FIN-02015 Espoo, Finland b
c
Received 18 August 1999; accepted 2 October 1999
Abstract Platinum coatings were thermally prepared on glass substrates. Hexachloroplatinic acid was used as a starting material for platinum. The thermal behaviour of H 2 PtCl 6 in the absence and presence of organic additives was investigated by thermogravimetry. X-ray diffractograms indicated the presence of crystalline Pt deposits. The examination of the surface morphology of the Pt coating by atomic force microscopy revealed that the surface roughness of coatings increased with increasing number of coating cycles. The sheet resistance of Pt coatings decreased with an increase in the number of coating cycles. The twice-coated electrode showed a typical cyclic–voltammetric response of platinum in 0.5 M H 2 SO4 . The FeŽCN. 63yr4y couple was used as a suitable test system for the Pt-coating electrodes. On the basis of the results obtained, it can be concluded that at least three coating cycles are required to be applied in order to obtain an electrode of good electrochemical performance. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermal preparation; Platinum coating; Atomic force microscopy; X-ray diffraction; Thermogravimetry; Cyclic voltammetry
1. Introduction Platinum thin films have been frequently employed in microelectronics, for example, as ohmic contacts in semiconductor devices w1,2x. This is due to the high electrical conductivity and chemical inertness of platinum films w3x. Platinum is also a suitable material for high-temperature applications because of its thermal stability. )
373.
Corresponding author. Tel.: q358-9-4511; fax: q358-9-462-
Several techniques can be employed for the deposition of platinum films w3x. Thermal evaporation and sputtering are the preferred methods of deposition. For certain applications where a substrate of special shape needs to be covered by Pt film, chemical vapour deposition ŽCVD. appears to be an advantageous technique w1x. Especially now that metalorganic CVD of Pt films has recently gained growing interest w1,4x. The quality of the Pt films depends mainly on the substrate temperature, deposition rate and chemical composition of the starting material. In many cases, a problem of adhesion is encountered
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 8 9 - 4
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K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
because of a weak interaction between platinum film and substrate w5x. An alternative method to prepare Pt coatings is based on the thermal decomposition of platinum precursors dissolved in a coating solution w6x. The thermal decomposition method is rather simple and enables one to prepare Pt coatings in a relatively large thickness range. Thin platinum films and coatings can also be used as indicator electrodes in electrochemical sensors w7x. For this purpose a high catalytic activity of the electrodes is needed. Recently, the electrochemical behaviour of thin Pt films of varying roughness was studied by Aschauer et al. w8x. In a previous paper, we reported on the application of thermally prepared Pt coatings in electrochemical sensors w9x. In the present study the preparation procedure has been further modified, which enables us to produce Pt coatings more easily. The surface morphology and electrochemical behaviour of the Pt coatings have been also investigated.
2. Experimental Platinum coatings were prepared on glass substrates by the thermal decomposition method based on our earlier experiments w9x. Pieces of soda-lime and Corning glass Ž1 = 5 cm2 , 1-mm thick. were used as substrates. The components of the coating solution were the same as that described in our previous study w9x. The preparation procedure is as follows: a chosen amount of the coating solution was applied onto a substrate surface by micropipette Žusually 5 mlrcm2 in a coating cycle.. After drying for 5 min at room temperature, the coated samples were placed in a muffle furnace. Annealing temperatures up to 6508C and 7008C for soda-lime and Corning glass were used, respectively. A series of Pt coatings on glass were produced by applying up to six coating cycles. The main difference in the preparation procedure in comparison to our previous report is that in this study we used thermal annealing in a muffle furnace instead of flame annealing. The proposed method is very simple and more reproducible and therefore better suited for practical application than that of flame annealing.
The structure and morphology of Pt coatings were investigated by X-ray diffraction ŽXRD. and atomic force microscopy ŽAFM.. The XRD measurements were carried out in a Philips MPD 1880 X-ray diffractometer using Cu K a radiation and an angular scan speed of 0.0282Qrs. The surface of Pt coatings was observed by AFM using a Nanoscope III system ŽDigital Instruments. operating in tapping mode ŽTM.. Cantilevers with integrated silicon tips were employed and all images were acquired at a scan rate of 1–2 Hz and a resolution of 512 = 512 pixels. To obtain representative images of the surface, wide areas Žtypically 20 mm. were scanned at different parts of the sample before the scan size was reduced to 2 = 2 mm to obtain the final image. From each image the root–mean–squared Žrms. roughness was calculated. The AFM images in this paper are depicted mainly as three-dimensional representations. For the thermoanalytical studies, a Seiko simultaneous TGrDTA 320 instrument was used. The sample mass for the H 2 PtCl 6 Ø xH 2 O complex Žs A. was 48.2 mg. In order to simulate the thermal Pt film deposition process, a TGrDTA curve was also recorded for a mixture Žs B., which contained the Pt complex together with the organic phase as in the coating solution. The sample B was dryed at 2008C to a constant weight Ž14.7 mg. before the thermoanalytical study. The heating rate was 58C miny1 except for A in the temperature interval 100–2008C where it was lowered to 18C miny1 because of the disturbances caused by water release. All measurements were carried out in flowing air Ž80 ml miny1 . with alumina as DTA reference material. To measure the sheet resistance of Pt coatings, an experimental setup consisting of two Pt wires Ž0.5mm thick. was used. The wires were tightly pressed onto the surface to form a square. Electrochemical measurements were performed in a three-electrode cell. The electrodes were polarized by an Autolab potentiostatrgalvanostat PGSTAT10 ŽEco Chemie, The Netherlands.. The potentiostat was controlled by the General Purpose Electrochemical Software ŽGPES. system. Platinum foil was used as a counter electrode and saturated calomel electrode ŽSCE. served as a reference electrode. All potentials are given with respect to the SCE. The working electrode area submerged into a solution was 3 cm2 . Cyclic–voltammetric experiments were conducted in
K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
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Fig. 1. XRD patterns for a series of Ptrglass samples annealed at 6258C. Number of coating cycles: Ža. 1, Žb. 2, Žc. 3, Žd. 4 and Že. 5.
an Ar-saturated 0.5 M H 2 SO4 solution at a potential scan rate of 50 mVrs. The solution of 0.5 M H 2 SO4 was prepared from analytical grade sulfuric acid ŽMerck. and Milli-Q water. The suitability of Pt coatings to be used for electroanalytical purposes was tested in the solution of 0.5 M KCl containing 1 mM K 4 FeŽCN. 6 . All solutions were saturated with argon gas Ž99.99%, AGA..
3. Results The coating procedure was optimized on the basis of XRD data. The degree of crystallinity and possible texture were tested for all coatings prepared. No
Fig. 2. TG curves for the H 2 PtCl 6 Ø xH 2 O complex ŽA. and for a mixture ŽB. which contained the Pt precursor and organic additives.
diffraction peaks were obtained when the annealing temperature was lower than 2008C. A typical diffraction pattern of platinum was obtained for the samples annealed at 3008C and higher temperatures. However, the adherence of Pt coatings to glass was very poor for the samples annealed at moderate temperatures Ž300–4008C.. The coatings could be simply swept away by a tissue paper. To increase adhesion, a higher annealing temperature is needed. Fig. 1 presents the diffraction profile for a series of Ptrglass samples annealed at 6258C. As can be seen in the figure, the intensity of peaks increases with an increase in the number of coating cycles. All five reflections of Pt were clearly seen in the range of 2Q
Fig. 3. The sheet resistance for a series of Ptrglass samples annealed at 6258C.
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angles from 20 to 90 even for the sample coated by a single cycle only. As expected, no other diffraction lines were present. The PtŽ111. reflection Žat 39.882Q . showed the highest intensity. The coatings annealed at temperatures higher than 6008C were stable and did not peel off from the substrate in the solution. The platinum precursor corresponding to formula H 2 PtCl 6 Ø xH 2 O Žsample A in Fig. 2. starts to decompose by a slow dehydration process already below 1008C. A faster release of water accompanied with HCl evolution follows then in the temperature
range 100–1708C. Due to the strong gas evolution in small bursts, the DTA curve becomes restless and is of little value in discerning any possible intermediate steps. In the range 170–3008C, a small weight loss is observed with a DTA maximum at 2508C. The composition of the sample at this point corresponds to PtCl 4 , as calculated from the final residue. The next two endothermic steps are more distinct. At 3308C, a weight plateau is reached corresponding to PtCl 2 , which is stable up to 4708C. The last decomposition step yields platinum metal as final residue above
Fig. 4. TM–AFM images of Pt coatings on Corning glass annealed at 7008C after Ža. 1, Žb. 2, Žc. 4 and Žd. 5 cycles. Image size: 2 = 2 mm. Depth scale: 50 nm from black to white.
K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
5308C. The total weight loss of 65.2% indicates that the original precursor contained 8 mol of water, i.e., had the composition H 2 PtCl 6 Ø 8H 2 O. Our observations on the mechanism and temperatures of the decomposition agree well with the previous studies where Kinoshita et al. w10x had studied a sample with x s 4.4 H 2 O while Hernandez and Choren w11x had ´ assumed their sample to be a hexahydrate. When the sample contained organic material, there was a dramatic change in thermal behaviour as seen in curve B of Fig. 2. Due to the burning of organics, the overall effect was strongly exothermic and the platinum residue was formed already at temperatures slightly above 4008C. A similar effect was observed also by Comninellis and Vercesi w6x when they compared the behaviour of pure H 2 PtCl 6 Ø 6H 2 O with that in alcoholic solution. The electrical properties of Pt coatings were investigated by measuring a sheet resistance Ž R s .. The experimental setup was similar to that described in Ref. w12x. Fig. 3 shows a representative sheet resistance curve for a series of Ptrglass samples. A single-time coated sample is of relatively high resistance. It should be noted that the scatter in the data for this coating is rather large, ranging from 30 to 80 V sqy1 . This can be explained by a non-uniform coverage of the substrate by platinum in a large scale. Usually, the area under the measurement was 1 cm2 . The resistance dropped significantly when the second coating cycle was made ŽFig. 3.. Further increase in the number of coating cycles leads to a gradual decrease in resistance. This is reasonable to consider on the basis of the following equation R s s rrd, where r is the resistivity and d is the thickness of a coating. Therefore, the film resistance is inversely proportional to the coating thickness. For a five-times-coated sample, an average R s was ca. 2 V sqy1 . In the AFM investigation, the morphological effects caused by the number of Pt coating cycles and annealing temperature were studied in order to obtain information on the quality and homogeneity of the Pt overlayers. It would be expected that with polycrystalline coatings on amorphous, flat surfaces the rms-roughness would increase with an increase of coating cycles. This was the general trend also with our samples as seen in the series of one to five coatings
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prepared at 7008C on Corning glass ŽFig. 4a–d.. The rms-roughness increased from 6 to 23 nm for once and five-times coated samples, respectively ŽFig. 5.. At 508C lower temperature on soda-lime glass substrate, the increase in roughness was not so pronounced, but the trend was still the same. Also, the homogeneity at 6508C appeared better ŽFig. 6a–e.. On the other hand, when the temperature was dropped further by 1008C to 5508C, a hole filling phenomenon was observed on soda lime glass substrate leading to a decrease in rms-roughness from 12 to 6 nm when the number of cycles was increased from three to four. Thus, the deposition temperature plays an important role in optimising the process for smoothness and uniformity. The electrochemical characterization of the Pt coatings was started with cyclic voltammetry in Arsaturated 0.5 M H 2 SO4 . Potential cycling in this solution was also used to remove the surface contamination from the coatings. However, it is worth to mention that the samples were fairly clean after thermal annealing as the temperature was high enough to burn all carbon compounds. The shape of the cyclic voltammograms of singly coated electrodes was remarkably different from the others ŽFig. 7.. This is specially evident in the hydrogen adsorption region where the peaks were strongly shifted. The surface oxide reduction peak was shifted to more negative potentials for this electrode. Such a behaviour seems to be caused by a high electrode resistance. The CVs of the electrodes of thicker
Fig. 5. The rms-roughness of the Pt coatings on soda-lime Ž6508C. and Corning glass Ž7008C. vs. the number of coating cycles.
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K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
Fig. 7. Cyclic voltammograms for Pt-coating electrodes in Arsaturated 0.5 M H 2 SO4 at a sweep rate of 50 mVrs. Electrode area 3 cm2 . The number of coating cycles increases consecutively from one to six.
coatings ŽFig. 7, curves 2–6. exhibit a typical response of polycrystalline platinum electrode w13x. Even the third discrete peak appeared between the two major ones in the anodic branch of the cyclovoltammogram. The quality of the CV curves improved for the coatings of higher thickness. For example, the separation of the potentials of the corresponding anodic and cathodic peaks decreased with the increase in the number of coating cycles Žsee the hydrogen adsorption region in Fig. 7.. We attribute this tendency to an increased conductance of the coatings of higher thickness. The roughness factor of Pt coatings Žthe ratio of real to geometric electrode area. was determined from the charge integrated in the area under the hydrogen desorption curve. As expected, the roughness factor Ž f r . increased with an increase in the number of coating cycles. An average f r value for a six-times-coated electrode was close to ten. The high f r value is not advantageous for the electrodes to be used for electroanalytical purposes. A direct consequence is that the background level increases and the signal to noise ratio becomes less favourable. The electrochemical behaviour of the Fe– ŽCN. 63yr4y couple was used to test the suitability of
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Fig. 8. Cyclic voltammograms for Pt-coating electrodes in Arsaturated 0.5 M KCl containing 1 mM K 4 FeŽCN. 6 . Sweep rate of 10 mVrs. Electrode area 3 cm2 . Ž1. one coating cycle, Ž2. two coating cycles and Ž3. six coating cycles.
Pt coatings as indicator electrodes in electroanalysis. It can be seen that the maximum current of anodic and cathodic peaks is relatively low for a singly coated electrode ŽFig. 8, curve 1.. For this electrode the f r value was close to unity. Therefore, we consider that there can be areas on the surface which are not covered by the coating metal or where the platinum deposit exist as separated islands. The other factor that changes the shape of the CV curves is the coating resistance. An increase in the number of coating cycles improves the shape of the voltammograms. The separation of anodic and cathodic peak potentials Ž D E s Epa y Epc . decreases and the factor Ep y Epr2 exhibits a value close to that of the reversible wave Ž2.2RTrF., where Ep is a peak potential and Epr2 a half-peak potential. The experimental Ep y Epr2 value varied between 53 and 58 mV for three or higher times coated electrodes. This value was significantly higher for a single-time-coated electrode. A general tendency is that the separation of peak potentials increases with increasing scan rate Ž Õ .. Even for a six-times-coated electrode the Ep is slightly dependent on Õ Žsee Fig. 9.. This can be attributed at least in part to the uncompensated solu-
Fig. 6. TM–AFM images of Pt coatings on soda-lime glass annealed at 6508C after Ža. 1, Žb. 2, Žc. 3, Žd. 4 and Že. 5 cycles. Image size: 2 = 2 mm. Depth scale: 50 nm from black to white.
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coating procedure was optimized and up to six coating cycles were applied. AFM images indicated an increase in the surface roughness with increase in the number of coatings cycles. At least three coating cycles should be applied for a reliable application of Pt-coating electrodes in electrochemical detection. These electrodes show a typical electrochemical behaviour of polycrystalline platinum and can be used as indicator electrodes in electrochemical sensors. Acknowledgements
Fig. 9. Cyclic voltammograms for a six-times-coated Pt electrode in Ar-saturated 0.5 M KCl solution containing 1 mM K 4 FeŽCN. 6 . Sweep rate: Ž1. 10, Ž2. 20, Ž3. 50 and Ž4. 100 mVrs.
tion resistance. The baseline-corrected peak current is linearly proportional to the square root of scan rate. The slope of the experimental i p vs. Õ 1r2 plot Ž2.52 mA s1r2 Õy1r2 . is close to that of the theoretically calculated one assuming a reversible charge transfer process. The diffusion coefficient of FeŽCN. 64y was taken as 6.3 = 10y6 cm2rs w14x. For the electrodes of relatively high coating thickness the separation of peak potentials was ca. 62–63 mV when a small sweep rate was used ŽFig. 9.. The expected D E value for a reversible wave is 59 mV Ž2.3RTrF. at 258C w15x. The slight difference could be attributed to the uncompensated solution resistance. Although the kinetics of the electron transfer reaction of the FeŽCN. 63yr4y couple on platinum is considered to be fast Ž k o is of the order of 0.1 cmrs w14x. still some kinetic effect can be present. The D Ep value increased considerably when the electrodes of lower Pt coating thickness were used and high sweep rates were applied. As mentioned above, this is mainly attributed to the resistance of the electrodes.
4. Conclusions A thermal decomposition method was used to prepare platinum coatings on glass substrates. The
This work was supported by the Estonian Science Foundation ŽGrant No. 3936.. One of us ŽK.T.. wants to thank the Finnish Centre for International Mobility ŽCIMO. for a research grant. Ms. Satu Harkonen is thanked for the TG measurements. ¨ ¨ References w1x A.A. Zinn, L. Brandt, H.D. Kaesz, R.F. Hicks, in: T.T. Kodas, M.J. Hampden-Smith ŽEds.., The Chemistry of Metal CVD, VCH, New York, 1994, p. 329. w2x D.C. Ivey, Plat. Met. Rev. 43 Ž1999. 2. w3x J. Chevallier, Thin Solid Films 40 Ž1977. 223. w4x J.M. Lee, C.S. Hwang, H.J. Cho, C.-G. Suk, H.J. Kim, J. Electrochem. Soc. 145 Ž1998. 1066. w5x M. Josowicz, J. Janata, M. Levy, J. Electrochem. Soc. 135 Ž1988. 112. w6x Ch. Comninellis, G.P. Vercesi, J. Appl. Electrochem. 21 Ž1991. 136. w7x G.C. Fiaccabrino, M. Koudelka-Hep, Electroanalysis 10 Ž1998. 217. w8x E. Aschauer, R. Fasching, M. Varahram, G. Jobst, G. Urban, G. Nicolussi, W. Husinsky, G. Friedbacher, M. Grasserbauer, J. Electroanal. Chem. 426 Ž1997. 157. w9x K. Tammeveski, T. Kikas, T. Tenno, L. Niinisto, ¨ Sensors and Actuators B 47 Ž1998. 21. w10x K. Kinoshita, K. Routsis, J.A.S. Bett, Thermochim. Acta 10 Ž1974. 109. w11x J.O. Hernandez, E.A. Choren, Thermochim. Acta 71 Ž1983. ´ 265. w12x J. Mizsei, P. Sipila, ¨ V. Lantto, Sensors and Actuators B 47 Ž1998. 139. w13x H. Angerstein-Kozlowska, in: E. Yeager, J.O’M. Bockris, B.E. Conway, S. Sarangapani ŽEds.., Comprehensive Treatise of Electrochemistry, Vol. 9, Plenum Press, New York, 1984, p. 15. w14x F. Kitamura, N. Nanbu, T. Ohsaka, K. Tokuda, J. Electroanal. Chem. 456 Ž1997. 113. w15x A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980.
Thermal preparation of thin platinum coatings and their electrochemical and atomic force microscopic characterization K. Tammeveski a , T. Tenno a , J. Niinisto¨ b,c , T. Leitner b, G. Friedbacher b, L. Niinisto¨ c,) a
Institute of Physical Chemistry, UniÕersity of Tartu, Jakobi 2, 51014 Tartu, Estonia Institute of Analytical Chemistry, Vienna UniÕersity of Technology, Getreidemarkt 9 r 151 A-1060 Vienna, Austria Laboratory of Inorganic and Analytical Chemistry, Helsinki UniÕersity of Technology, P.O. Box 6100, FIN-02015 Espoo, Finland b
c
Received 18 August 1999; accepted 2 October 1999
Abstract Platinum coatings were thermally prepared on glass substrates. Hexachloroplatinic acid was used as a starting material for platinum. The thermal behaviour of H 2 PtCl 6 in the absence and presence of organic additives was investigated by thermogravimetry. X-ray diffractograms indicated the presence of crystalline Pt deposits. The examination of the surface morphology of the Pt coating by atomic force microscopy revealed that the surface roughness of coatings increased with increasing number of coating cycles. The sheet resistance of Pt coatings decreased with an increase in the number of coating cycles. The twice-coated electrode showed a typical cyclic–voltammetric response of platinum in 0.5 M H 2 SO4 . The FeŽCN. 63yr4y couple was used as a suitable test system for the Pt-coating electrodes. On the basis of the results obtained, it can be concluded that at least three coating cycles are required to be applied in order to obtain an electrode of good electrochemical performance. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermal preparation; Platinum coating; Atomic force microscopy; X-ray diffraction; Thermogravimetry; Cyclic voltammetry
1. Introduction Platinum thin films have been frequently employed in microelectronics, for example, as ohmic contacts in semiconductor devices w1,2x. This is due to the high electrical conductivity and chemical inertness of platinum films w3x. Platinum is also a suitable material for high-temperature applications because of its thermal stability. )
373.
Corresponding author. Tel.: q358-9-4511; fax: q358-9-462-
Several techniques can be employed for the deposition of platinum films w3x. Thermal evaporation and sputtering are the preferred methods of deposition. For certain applications where a substrate of special shape needs to be covered by Pt film, chemical vapour deposition ŽCVD. appears to be an advantageous technique w1x. Especially now that metalorganic CVD of Pt films has recently gained growing interest w1,4x. The quality of the Pt films depends mainly on the substrate temperature, deposition rate and chemical composition of the starting material. In many cases, a problem of adhesion is encountered
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 8 9 - 4
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K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
because of a weak interaction between platinum film and substrate w5x. An alternative method to prepare Pt coatings is based on the thermal decomposition of platinum precursors dissolved in a coating solution w6x. The thermal decomposition method is rather simple and enables one to prepare Pt coatings in a relatively large thickness range. Thin platinum films and coatings can also be used as indicator electrodes in electrochemical sensors w7x. For this purpose a high catalytic activity of the electrodes is needed. Recently, the electrochemical behaviour of thin Pt films of varying roughness was studied by Aschauer et al. w8x. In a previous paper, we reported on the application of thermally prepared Pt coatings in electrochemical sensors w9x. In the present study the preparation procedure has been further modified, which enables us to produce Pt coatings more easily. The surface morphology and electrochemical behaviour of the Pt coatings have been also investigated.
2. Experimental Platinum coatings were prepared on glass substrates by the thermal decomposition method based on our earlier experiments w9x. Pieces of soda-lime and Corning glass Ž1 = 5 cm2 , 1-mm thick. were used as substrates. The components of the coating solution were the same as that described in our previous study w9x. The preparation procedure is as follows: a chosen amount of the coating solution was applied onto a substrate surface by micropipette Žusually 5 mlrcm2 in a coating cycle.. After drying for 5 min at room temperature, the coated samples were placed in a muffle furnace. Annealing temperatures up to 6508C and 7008C for soda-lime and Corning glass were used, respectively. A series of Pt coatings on glass were produced by applying up to six coating cycles. The main difference in the preparation procedure in comparison to our previous report is that in this study we used thermal annealing in a muffle furnace instead of flame annealing. The proposed method is very simple and more reproducible and therefore better suited for practical application than that of flame annealing.
The structure and morphology of Pt coatings were investigated by X-ray diffraction ŽXRD. and atomic force microscopy ŽAFM.. The XRD measurements were carried out in a Philips MPD 1880 X-ray diffractometer using Cu K a radiation and an angular scan speed of 0.0282Qrs. The surface of Pt coatings was observed by AFM using a Nanoscope III system ŽDigital Instruments. operating in tapping mode ŽTM.. Cantilevers with integrated silicon tips were employed and all images were acquired at a scan rate of 1–2 Hz and a resolution of 512 = 512 pixels. To obtain representative images of the surface, wide areas Žtypically 20 mm. were scanned at different parts of the sample before the scan size was reduced to 2 = 2 mm to obtain the final image. From each image the root–mean–squared Žrms. roughness was calculated. The AFM images in this paper are depicted mainly as three-dimensional representations. For the thermoanalytical studies, a Seiko simultaneous TGrDTA 320 instrument was used. The sample mass for the H 2 PtCl 6 Ø xH 2 O complex Žs A. was 48.2 mg. In order to simulate the thermal Pt film deposition process, a TGrDTA curve was also recorded for a mixture Žs B., which contained the Pt complex together with the organic phase as in the coating solution. The sample B was dryed at 2008C to a constant weight Ž14.7 mg. before the thermoanalytical study. The heating rate was 58C miny1 except for A in the temperature interval 100–2008C where it was lowered to 18C miny1 because of the disturbances caused by water release. All measurements were carried out in flowing air Ž80 ml miny1 . with alumina as DTA reference material. To measure the sheet resistance of Pt coatings, an experimental setup consisting of two Pt wires Ž0.5mm thick. was used. The wires were tightly pressed onto the surface to form a square. Electrochemical measurements were performed in a three-electrode cell. The electrodes were polarized by an Autolab potentiostatrgalvanostat PGSTAT10 ŽEco Chemie, The Netherlands.. The potentiostat was controlled by the General Purpose Electrochemical Software ŽGPES. system. Platinum foil was used as a counter electrode and saturated calomel electrode ŽSCE. served as a reference electrode. All potentials are given with respect to the SCE. The working electrode area submerged into a solution was 3 cm2 . Cyclic–voltammetric experiments were conducted in
K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
137
Fig. 1. XRD patterns for a series of Ptrglass samples annealed at 6258C. Number of coating cycles: Ža. 1, Žb. 2, Žc. 3, Žd. 4 and Že. 5.
an Ar-saturated 0.5 M H 2 SO4 solution at a potential scan rate of 50 mVrs. The solution of 0.5 M H 2 SO4 was prepared from analytical grade sulfuric acid ŽMerck. and Milli-Q water. The suitability of Pt coatings to be used for electroanalytical purposes was tested in the solution of 0.5 M KCl containing 1 mM K 4 FeŽCN. 6 . All solutions were saturated with argon gas Ž99.99%, AGA..
3. Results The coating procedure was optimized on the basis of XRD data. The degree of crystallinity and possible texture were tested for all coatings prepared. No
Fig. 2. TG curves for the H 2 PtCl 6 Ø xH 2 O complex ŽA. and for a mixture ŽB. which contained the Pt precursor and organic additives.
diffraction peaks were obtained when the annealing temperature was lower than 2008C. A typical diffraction pattern of platinum was obtained for the samples annealed at 3008C and higher temperatures. However, the adherence of Pt coatings to glass was very poor for the samples annealed at moderate temperatures Ž300–4008C.. The coatings could be simply swept away by a tissue paper. To increase adhesion, a higher annealing temperature is needed. Fig. 1 presents the diffraction profile for a series of Ptrglass samples annealed at 6258C. As can be seen in the figure, the intensity of peaks increases with an increase in the number of coating cycles. All five reflections of Pt were clearly seen in the range of 2Q
Fig. 3. The sheet resistance for a series of Ptrglass samples annealed at 6258C.
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K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
angles from 20 to 90 even for the sample coated by a single cycle only. As expected, no other diffraction lines were present. The PtŽ111. reflection Žat 39.882Q . showed the highest intensity. The coatings annealed at temperatures higher than 6008C were stable and did not peel off from the substrate in the solution. The platinum precursor corresponding to formula H 2 PtCl 6 Ø xH 2 O Žsample A in Fig. 2. starts to decompose by a slow dehydration process already below 1008C. A faster release of water accompanied with HCl evolution follows then in the temperature
range 100–1708C. Due to the strong gas evolution in small bursts, the DTA curve becomes restless and is of little value in discerning any possible intermediate steps. In the range 170–3008C, a small weight loss is observed with a DTA maximum at 2508C. The composition of the sample at this point corresponds to PtCl 4 , as calculated from the final residue. The next two endothermic steps are more distinct. At 3308C, a weight plateau is reached corresponding to PtCl 2 , which is stable up to 4708C. The last decomposition step yields platinum metal as final residue above
Fig. 4. TM–AFM images of Pt coatings on Corning glass annealed at 7008C after Ža. 1, Žb. 2, Žc. 4 and Žd. 5 cycles. Image size: 2 = 2 mm. Depth scale: 50 nm from black to white.
K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
5308C. The total weight loss of 65.2% indicates that the original precursor contained 8 mol of water, i.e., had the composition H 2 PtCl 6 Ø 8H 2 O. Our observations on the mechanism and temperatures of the decomposition agree well with the previous studies where Kinoshita et al. w10x had studied a sample with x s 4.4 H 2 O while Hernandez and Choren w11x had ´ assumed their sample to be a hexahydrate. When the sample contained organic material, there was a dramatic change in thermal behaviour as seen in curve B of Fig. 2. Due to the burning of organics, the overall effect was strongly exothermic and the platinum residue was formed already at temperatures slightly above 4008C. A similar effect was observed also by Comninellis and Vercesi w6x when they compared the behaviour of pure H 2 PtCl 6 Ø 6H 2 O with that in alcoholic solution. The electrical properties of Pt coatings were investigated by measuring a sheet resistance Ž R s .. The experimental setup was similar to that described in Ref. w12x. Fig. 3 shows a representative sheet resistance curve for a series of Ptrglass samples. A single-time coated sample is of relatively high resistance. It should be noted that the scatter in the data for this coating is rather large, ranging from 30 to 80 V sqy1 . This can be explained by a non-uniform coverage of the substrate by platinum in a large scale. Usually, the area under the measurement was 1 cm2 . The resistance dropped significantly when the second coating cycle was made ŽFig. 3.. Further increase in the number of coating cycles leads to a gradual decrease in resistance. This is reasonable to consider on the basis of the following equation R s s rrd, where r is the resistivity and d is the thickness of a coating. Therefore, the film resistance is inversely proportional to the coating thickness. For a five-times-coated sample, an average R s was ca. 2 V sqy1 . In the AFM investigation, the morphological effects caused by the number of Pt coating cycles and annealing temperature were studied in order to obtain information on the quality and homogeneity of the Pt overlayers. It would be expected that with polycrystalline coatings on amorphous, flat surfaces the rms-roughness would increase with an increase of coating cycles. This was the general trend also with our samples as seen in the series of one to five coatings
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prepared at 7008C on Corning glass ŽFig. 4a–d.. The rms-roughness increased from 6 to 23 nm for once and five-times coated samples, respectively ŽFig. 5.. At 508C lower temperature on soda-lime glass substrate, the increase in roughness was not so pronounced, but the trend was still the same. Also, the homogeneity at 6508C appeared better ŽFig. 6a–e.. On the other hand, when the temperature was dropped further by 1008C to 5508C, a hole filling phenomenon was observed on soda lime glass substrate leading to a decrease in rms-roughness from 12 to 6 nm when the number of cycles was increased from three to four. Thus, the deposition temperature plays an important role in optimising the process for smoothness and uniformity. The electrochemical characterization of the Pt coatings was started with cyclic voltammetry in Arsaturated 0.5 M H 2 SO4 . Potential cycling in this solution was also used to remove the surface contamination from the coatings. However, it is worth to mention that the samples were fairly clean after thermal annealing as the temperature was high enough to burn all carbon compounds. The shape of the cyclic voltammograms of singly coated electrodes was remarkably different from the others ŽFig. 7.. This is specially evident in the hydrogen adsorption region where the peaks were strongly shifted. The surface oxide reduction peak was shifted to more negative potentials for this electrode. Such a behaviour seems to be caused by a high electrode resistance. The CVs of the electrodes of thicker
Fig. 5. The rms-roughness of the Pt coatings on soda-lime Ž6508C. and Corning glass Ž7008C. vs. the number of coating cycles.
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K. TammeÕeski et al.r Applied Surface Science 156 (2000) 135–142
Fig. 7. Cyclic voltammograms for Pt-coating electrodes in Arsaturated 0.5 M H 2 SO4 at a sweep rate of 50 mVrs. Electrode area 3 cm2 . The number of coating cycles increases consecutively from one to six.
coatings ŽFig. 7, curves 2–6. exhibit a typical response of polycrystalline platinum electrode w13x. Even the third discrete peak appeared between the two major ones in the anodic branch of the cyclovoltammogram. The quality of the CV curves improved for the coatings of higher thickness. For example, the separation of the potentials of the corresponding anodic and cathodic peaks decreased with the increase in the number of coating cycles Žsee the hydrogen adsorption region in Fig. 7.. We attribute this tendency to an increased conductance of the coatings of higher thickness. The roughness factor of Pt coatings Žthe ratio of real to geometric electrode area. was determined from the charge integrated in the area under the hydrogen desorption curve. As expected, the roughness factor Ž f r . increased with an increase in the number of coating cycles. An average f r value for a six-times-coated electrode was close to ten. The high f r value is not advantageous for the electrodes to be used for electroanalytical purposes. A direct consequence is that the background level increases and the signal to noise ratio becomes less favourable. The electrochemical behaviour of the Fe– ŽCN. 63yr4y couple was used to test the suitability of
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Fig. 8. Cyclic voltammograms for Pt-coating electrodes in Arsaturated 0.5 M KCl containing 1 mM K 4 FeŽCN. 6 . Sweep rate of 10 mVrs. Electrode area 3 cm2 . Ž1. one coating cycle, Ž2. two coating cycles and Ž3. six coating cycles.
Pt coatings as indicator electrodes in electroanalysis. It can be seen that the maximum current of anodic and cathodic peaks is relatively low for a singly coated electrode ŽFig. 8, curve 1.. For this electrode the f r value was close to unity. Therefore, we consider that there can be areas on the surface which are not covered by the coating metal or where the platinum deposit exist as separated islands. The other factor that changes the shape of the CV curves is the coating resistance. An increase in the number of coating cycles improves the shape of the voltammograms. The separation of anodic and cathodic peak potentials Ž D E s Epa y Epc . decreases and the factor Ep y Epr2 exhibits a value close to that of the reversible wave Ž2.2RTrF., where Ep is a peak potential and Epr2 a half-peak potential. The experimental Ep y Epr2 value varied between 53 and 58 mV for three or higher times coated electrodes. This value was significantly higher for a single-time-coated electrode. A general tendency is that the separation of peak potentials increases with increasing scan rate Ž Õ .. Even for a six-times-coated electrode the Ep is slightly dependent on Õ Žsee Fig. 9.. This can be attributed at least in part to the uncompensated solu-
Fig. 6. TM–AFM images of Pt coatings on soda-lime glass annealed at 6508C after Ža. 1, Žb. 2, Žc. 3, Žd. 4 and Že. 5 cycles. Image size: 2 = 2 mm. Depth scale: 50 nm from black to white.
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coating procedure was optimized and up to six coating cycles were applied. AFM images indicated an increase in the surface roughness with increase in the number of coatings cycles. At least three coating cycles should be applied for a reliable application of Pt-coating electrodes in electrochemical detection. These electrodes show a typical electrochemical behaviour of polycrystalline platinum and can be used as indicator electrodes in electrochemical sensors. Acknowledgements
Fig. 9. Cyclic voltammograms for a six-times-coated Pt electrode in Ar-saturated 0.5 M KCl solution containing 1 mM K 4 FeŽCN. 6 . Sweep rate: Ž1. 10, Ž2. 20, Ž3. 50 and Ž4. 100 mVrs.
tion resistance. The baseline-corrected peak current is linearly proportional to the square root of scan rate. The slope of the experimental i p vs. Õ 1r2 plot Ž2.52 mA s1r2 Õy1r2 . is close to that of the theoretically calculated one assuming a reversible charge transfer process. The diffusion coefficient of FeŽCN. 64y was taken as 6.3 = 10y6 cm2rs w14x. For the electrodes of relatively high coating thickness the separation of peak potentials was ca. 62–63 mV when a small sweep rate was used ŽFig. 9.. The expected D E value for a reversible wave is 59 mV Ž2.3RTrF. at 258C w15x. The slight difference could be attributed to the uncompensated solution resistance. Although the kinetics of the electron transfer reaction of the FeŽCN. 63yr4y couple on platinum is considered to be fast Ž k o is of the order of 0.1 cmrs w14x. still some kinetic effect can be present. The D Ep value increased considerably when the electrodes of lower Pt coating thickness were used and high sweep rates were applied. As mentioned above, this is mainly attributed to the resistance of the electrodes.
4. Conclusions A thermal decomposition method was used to prepare platinum coatings on glass substrates. The
This work was supported by the Estonian Science Foundation ŽGrant No. 3936.. One of us ŽK.T.. wants to thank the Finnish Centre for International Mobility ŽCIMO. for a research grant. Ms. Satu Harkonen is thanked for the TG measurements. ¨ ¨ References w1x A.A. Zinn, L. Brandt, H.D. Kaesz, R.F. Hicks, in: T.T. Kodas, M.J. Hampden-Smith ŽEds.., The Chemistry of Metal CVD, VCH, New York, 1994, p. 329. w2x D.C. Ivey, Plat. Met. Rev. 43 Ž1999. 2. w3x J. Chevallier, Thin Solid Films 40 Ž1977. 223. w4x J.M. Lee, C.S. Hwang, H.J. Cho, C.-G. Suk, H.J. Kim, J. Electrochem. Soc. 145 Ž1998. 1066. w5x M. Josowicz, J. Janata, M. Levy, J. Electrochem. Soc. 135 Ž1988. 112. w6x Ch. Comninellis, G.P. Vercesi, J. Appl. Electrochem. 21 Ž1991. 136. w7x G.C. Fiaccabrino, M. Koudelka-Hep, Electroanalysis 10 Ž1998. 217. w8x E. Aschauer, R. Fasching, M. Varahram, G. Jobst, G. Urban, G. Nicolussi, W. Husinsky, G. Friedbacher, M. Grasserbauer, J. Electroanal. Chem. 426 Ž1997. 157. w9x K. Tammeveski, T. Kikas, T. Tenno, L. Niinisto, ¨ Sensors and Actuators B 47 Ž1998. 21. w10x K. Kinoshita, K. Routsis, J.A.S. Bett, Thermochim. Acta 10 Ž1974. 109. w11x J.O. Hernandez, E.A. Choren, Thermochim. Acta 71 Ž1983. ´ 265. w12x J. Mizsei, P. Sipila, ¨ V. Lantto, Sensors and Actuators B 47 Ž1998. 139. w13x H. Angerstein-Kozlowska, in: E. Yeager, J.O’M. Bockris, B.E. Conway, S. Sarangapani ŽEds.., Comprehensive Treatise of Electrochemistry, Vol. 9, Plenum Press, New York, 1984, p. 15. w14x F. Kitamura, N. Nanbu, T. Ohsaka, K. Tokuda, J. Electroanal. Chem. 456 Ž1997. 113. w15x A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980.