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Nano peel-off fabrication pattern with polymer overcoat layer on glassy carbon electrodes for Pt electrodeposition
Applied Surface Science 255 (2009) 9154–9158
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nano peel-off fabrication pattern with polymer overcoat layer on glassy carbon electrodes for Pt electrodeposition Akira Kishi, Minoru Umeda * Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 March 2009 Received in revised form 23 June 2009 Accepted 30 June 2009 Available online 4 July 2009
This article describes a new technique for fabricating an electrocatalyst model in which the particle size and interparticle distance are controlled independently. We designed a uniform insulating polymer layer as a mask on an electroconductive glassy carbon substrate and then peeled off a part of the layer in nanosized dots by scratching the overcoat layer using an atomic force microscope (AFM) cantilever. Pt particles electrodeposited only on the peeled off area of the glassy carbon. To peel-off a small area on the glassy carbon, a 29 2 nm thick insulating polymer overcoat layer and a cantilever operating area of 10 nm 10 nm were used, and the smallest peel-off area obtained was 30 nm 30 nm. Thereafter, we performed the peel-off procedure on the polymer overcoat layer of the glassy carbon substrate having a cantilever operating area of 80 nm 80 nm. Pt deposition of 100–150 nm in diameter was successfully achieved by adjusting the interparticle distance. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.07. b Keywords: Nano peel-off pattern fabrication Pt electrodeposition Glassy carbon electrode Polymer overcoat layer and atomic force microscopy
1. Introduction Metallic nano-sized dots are widely used in practical applications of electrocatalysts. Almost all of the electrocatalysts have a structure that loads nano-sized and zero-dimension particles on an electroconductive support. The particle size and interparticle distance are believed to play an important role in functional electrode catalysts [1–3]. A smaller particle size yields a larger surface area, which will supply a larger number of reaction sites. However, to the best of our knowledge there is no research report in the field of electrocatalysts, in which particle size and interparticle distance are independently controlled. With regard to the field of electrocatalysts, it is worthwhile to clarify the relationship between particle size and interparticle distance. To realize a typical electrocatalyst model in which the particle size and interparticle distance are controlled independently, a metallic nano fabrication on an electroconductive substrate seems to be an effective approach [4,5]. Then, we overview of the recent reports on metal nano fabrication. There are two metal nano fabrication techniques: maskless fabrication [6–16] and mask fabrication [17–34]. Maskless fabrication uses a dip pen [6–8], microcontact printing [9,10], metal deposition onto a sample using a charged surface potential [11–15], and an indent method [16].
* Corresponding author. Tel.: +81 258 47 9323; fax: +81 258 47 9300. E-mail address: [email protected] (M. Umeda). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.06.125
For mask fabrication, part of the mask prepared on a substrate is removed in lines by an electron beam [17–19], ion-beam [20–22], chemical etching [23–25], a atomic force microscope (AFM) cantilever [26–29], mold [30–34], etc., where the metallic lines are selectively deposited on the substrate in places where the mask was removed. However, till date, metallic nano fabrication studies have focused on electrode-pattern formation for semiconductor devices [8,10,12,14,15,17–20,23,25–31,34]. Therefore, the metallic nano fabrication has used a technique that deposits a single dimension line of metal on a semiconductor Si substrate, and there are no research reports on the deposition of zero-dimensional metal dots on an electroconductive substrate. To fabricate the electrocatalyst model, we focus our attention on a mask fabrication method that deposits metallic nano particles on the substrate. This method can deposit metallic particles that are of the same size as the area of the mask removed from arbitrary positions on the electroconductive substrate. One noteworthy mask fabrication method employs a line scratch technique that removes the mask using an AFM cantilever [26,27,29]. We intend to develop this line scratch method into a dot peel-off method. This new mask fabrication method is demonstrated in Fig. 1. In this work, an insulating polymer overcoat layer was first prepared by varying its thickness on an electroconductive glassy carbon substrate. Thereafter, the polymer overcoat layer is peeled off by a scratch technique using an AFM cantilever. In this process, the peel-off width and inter peel-off distance of the polymer
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Fig. 1. Schematic of the nano-patterning process: (a) a polymer over coat layer is formed on glassy carbon; (b) peel-off patterning of polymer overcoat layer using a cantilever of AFM; and (c) Pt electrodeposition.
overcoat layer are controlled by changing the operating width and inter peel-off distance of the cantilever. Then, Pt particles are electrodeposited only on the peeled off area of the glassy carbon. Using this procedure, the Pt electrocatalyst model for fuel cells is fabricated successfully.
2. Experimental 2.1. Preparation of glassy carbon with polymer overcoat A glassy carbon plate was purchased from Tokai Carbon Co., Ltd. (GC-20SS). The glassy carbon plate was cut into 1 cm2 pieces. One side of the glassy carbon plate was polished using a buff (Refinetec Co., Ltd., No. 55–308) and alumina (Union Carbide, 0.05 mm in diameter) suspension. Thereafter, the polished glassy carbon was rinsed using Milli-Q water and then dried. The roughness of the polished surface was measured using an AFM (Shimadzu, SPM-9500 J3), and the arithmetic average roughness (Ra) was around 2.2 nm. Next, the polished glassy carbon was immersed in a perfluorinated resin solution (Asahi glass Co., Ltd., Cytop, CTX-109, 2– 4.5 wt.%), and it was pulled up at a constant speed of 0.7 cm min 1 using a motorized micromanipulator (Narishige Co., Ltd., MM-80). After this dip coating procedure, the glassy carbon was dried at
150 8C in an oven chamber for 1 h to form a 20–115 nm thick polymer overcoat layer. 2.2. Micro and nano peel-off of the polymer overcoat The polymer overcoat layer was scratched by an AFM equipped with a Si cantilever (Nano World, PPP-NCHR) in a contact mode to peel-off the polymer overcoat layer. The scanning speed, loading force and number of repeated scanning of the cantilever were 2–1200 nm s 1, 0.6–4.3 mN and 2–5 times, respectively. After scratching, the sample was immersed for 5 min in Milli-Q water to brush off the polymer rubbish by ultrasonic cleaning. 2.3. Pt electrodeposition on the nano peel-off substrate The scratched sample was used as a working electrode for Pt electrodeposition. Ag/Ag2SO4 and Pt wires were used as reference and counter electrodes, respectively. Using a potentiostat (ALS, Model 802B), Pt electrodeposition was carried out five times in a solution mixture of 5 mmol dm 3 H2PtCl6 + 40 mmol dm 3 HCl for 0.1 s at an electrode potential of 0.6 V vs. Ag/Ag2SO4 with an interval of 5 s at 20 8C. Then, the sample was thoroughly rinsed in Milli-Q water and dried.
Fig. 2. Glassy carbon substrate having a polymer overcoat layer with an AFM-based 10 mm width scratch. (a) AFM topography; (b) a cross-sectional profile of the dashed line of (a); (c and d) EDS elemental mappings of F and C of (a).
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The sample surface was characterized by AFM and a scanning electron microscope (SEM) (JEOL, JSM-6060A) coupled with an energy dispersive X-ray spectroscopy (EDS) (JEOL, JED-2300). 3. Results and discussion 3.1. Micro-scale peel-off of the overcoat layer prepared on glassy carbon To realize the concept shown in Fig. 1, an insulating polymer overcoat layer about 100 nm thick was first formed on the glassy carbon substrate. The peel-off conditions of the overcoat layer were an operating area of 10.0 mm 10.0 mm, a scanning speed of 120 mm s 1, a loading force of 4.3 mN, and five repeated scannings of the cantilever. The surface topography of the resulting sample measured by AFM is shown in Fig. 2(a). The bright and dark parts of the figure correspond to the ridge and trench of the sample, respectively. A cross-sectional view of the area marked with a dotted line in Fig. 2(a) is shown in (b). From Fig. 2(b), it appears that the overcoat layer was removed from the cantilever operating area. Fig. 2(c and d) shows EDS elemental mapping of F and C of the same sample. In Fig. 2(c), an infinitesimal F-based signal originating in the perfluorinated polymer layer is observed in the cantilever operating area. Moreover, from Fig. 2(d), the same area of the glassy carbon is considered to be exposed, because the C signal from the glassy carbon is very high. These two results indicated that the insulating perfluorinated resin was perfectly peeled off, and the electroconductive glassy carbon was exposed in the cantilever operating area. Therefore the step height shown in Fig. 2(b) is believed to be equal to the thickness of the polymer overcoat layer. As a result, the thickness of the polymer overcoat layer is estimated to be 106 6 nm. 3.2. Relationship between a nano scale cantilever operating area and the peeled off area The cantilever operating area was downsized to a nano scale, and a relationship between an operating area and a peeled off area was investigated. In this experiment, a 41 4 nm thick insulating layer was formed on the glassy carbon. The thickness was obtained in a manner similar to that described for Fig. 1. Fig. 3(a) shows an AFM image of the sample after a 500 nm 500 nm cantilever operation. Fig. 3(b) shows a cross-sectional view of the area indicated by dotted line in Fig. 3(a). From Fig. 3(b), the dark 600 nm 600 nm area in Fig. 3(a) is known to be flat, and its average depth is 42 1 nm. On comparing the thickness of the polymer overcoat layer and the average depth of the scratched part, it was found that the insulating polymer overcoat layer was peeled off completely and the electroconductive glassy carbon appeared. Experiments similar to those shown in Fig. 3 were performed by changing the film thickness and the cantilever operating area. In Fig. 4, the results are shown in terms of a relationship between the cantilever operating width and peel-off width of the polymer overcoat layer as a function of the overcoat layer thickness. The dotted lines in the figure indicate a slope of 1.0. The peel-off width becomes narrower as the cantilever operating width is reduced. When the cantilever operating width is constant, the peel-off width of a 106 6 nm thick overcoat layer is greater than that of 29 2 and 41 4 nm thick layers. This can be explained by the cantilever operating procedure for a complete peel-off, which requires a number of repetitive scratching for a thicker overcoat layer [29]. Only for the 29 2 nm thick samples, even when the cantilever is operated at less than 100 nm, the peel-off width becomes proportionally narrow as the operating cantilever width is reduced. When
Fig. 3. (a) AFM topography of a glassy carbon substrate with a polymer overcoat layer after the cantilever peel-off. The thickness of the polymer overcoat layer was 41 4 nm and the operating area was 500 nm 500 nm. (b) Cross-sectional profile of the dashed line of (a).
the cantilever operation width is 10 nm, the narrowest peel-off width of 30 nm can be achieved. 3.3. Pt electrodeposition on nano peeled off patterns on the glassy carbon The cantilever scratches were conducted for a 41 4 nm thick insulating layer formed on the glassy carbon. Fig. 5(a) shows an AFM image of the peel-off sample, which has nine scratches prepared by an operating area of 100 nm 100 nm and an inter peel-off distance of 1.5 mm. In the figure, nine dark spots are observed with an interdistance of 1.5 mm. Fig. 5(b) shows a cross-sectional view of
Fig. 4. The operating width vs. peel-off width as a function of the polymer overcoat layer thickness. *: 106 6; : 41 4; : 29 2 nm.
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Fig. 5. (a) AFM image of the peel-off sample which has nine scratches prepared with an operating area of 100 nm 100 nm and an interdistance of 1.5 mm; (b) a crosssectional view of the dotted line of (a); (c) SEM image and (d) EDS elemental mapping of Pt after conducting Pt electrodeposition on the sample in (a).
the dotted line of Fig. 5(a). This figure shows that the depths of the scratched area were 42 1 nm. Clearly, the electroconductive glassy carbon is exposed, because the scratch depth and thickness of the polymer overcoat layer were almost the same.
Fig. 5(c and d) shows an SEM image and an EDS elemental mapping of the Pt after a Pt electrodeposition on the sample in Fig. 5(a). Comparing Fig. 5(a and c) shows that something is deposited in the peeled off area. This deposition is not a single
Fig. 6. (a) AFM image of the peel-off sample which has four scratches prepared with an operating area of 80 nm 80 nm and an interdistance of 500 nm; (b) a cross-sectional view of the dotted line of (a); (c) SEM image and (d) EDS elemental mapping of Pt after electrodeposition on the sample in (a).
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particle, but an aggregation of 50–100 nm thick particles. The EDS elemental mapping, as shown in Fig. 5(d), indicates that Pt electrodeposition took place. In addition, an infinitesimal Cl signal from H2PtCl6 for electrodeposition is observed from the EDS analysis. As a result, Pt particles were electrodeposited only on the exposed parts of the glassy carbon substrate. Next, the scratch was downsized using the sample having a 29 2 nm thick insulating layer on the glassy carbon. Fig. 6(a) shows an AFM image of the peeled off result, which has four scratches prepared by operating area of 80 nm 80 nm and an interdistance of 500 nm. In the figure, four dark spots are observed with an interval of 500 nm. Fig. 6(b) shows a cross-sectional view of the dotted line of Fig. 6(a), indicating that the scratch depths were 31 2 nm and the peel-off width was 100 nm. It is apparent that the electroconductive glassy carbon was exposed, because the scratch depth and thickness of the polymer overcoat layer are almost the same. Therefore, the sample in Fig. 6(a) successfully downsized the peel-off width and the interdistance, compared with the sample in Fig. 5(a). This achievement in downsizing is attributed to the fact that the thickness of the polymer overcoat layer of the sample in Fig. 6(a) was less than that of the sample in Fig. 5(a). Fig. 6(c and d) shows an SEM image and an EDS elemental mapping of Pt after Pt electrodeposition on the sample of Fig. 6(a). When we compare Fig. 6(a and c), some deposition is seen in the peeled off parts. In this experiment, the EDS elemental mapping, as shown in Fig. 6(d), indicates that Pt electrodeposition took place and no Cl-based signal was observed from the EDS analysis. As a result, Pt particles were electrodeposited only on the exposed parts of the glassy carbon substrate. Next, Pt electroplating was attempted for the 30 nm peeled off sample, resulted in no Pt deposition. Although the glassy carbon substrate is known to be exposed by the peel-off from the result shown in Fig. 6, the reason can be explained that the electrolytic solution is hard to contact with the electrode through the small holes. In conclusion, we succeeded in fabricating the Pt clusters, 100– 150 nm in diameter, with an interparticle distance of 500 nm, within limited areas of the glassy carbon substrate using a newly developed mask fabrication technique. In the future, we have a plan to do experiments on 30 nm width Pt by preparing a thinner insulating layer on the electrode substrate. 4. Conclusions We developed a new mask fabrication technique for fabricating nano-sized Pt particles at arbitrary positions on an electroconductive glassy carbon substrate. Portions of a polymer overcoat layer were peeled off by an AFM cantilever, and Pt particles were electrodeposited only on the peeled off parts of the glassy carbon. First, the peel-off procedure of the polymer overcoat layer prepared on the glassy carbon substrate was investigated. The cantilever operation perfectly peeled off the insulating perfluorinated resin and exposed the electroconductive glassy carbon. This enabled the estimation of the thickness of the polymer overcoat layer.
Second, we investigated the relationship between the cantilever operating width and the peel-off width of the overcoat layer as a function of its thickness. The resulting relationship showed that 30 nm was the narrowest width that could be peeled off, and it was achieved when the polymer overcoat layer was 29 2 nm thick and the cantilever operating width was 10 nm. Finally, for fabricating a Pt-dot-pattern electrode, a 100 nm wide peel-off pattern, with a 500 nm inter peel-off distance for a 29 2 nm thick overcoat layer was prepared. After electrodepositing Pt on the sample, Pt particles were deposited only on the parts of the substrate where the overcoat layer was removed. In conclusion, we successfully fabricated a Pt electrocatalyst model using a newly developed mask fabrication technique. Acknowledgement Part of this work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] H. Yano, J. Inukai, H. Uchida, M. Watanabe, P.K. Babu, T. Kobayashi, J.H. Chung, E. Oldfield, A. Wieckowski, Phys. Chem. Chem. Phys. 42 (2006) 4932. [2] L.C. Ordo´n˜ez, P. Roqueroc, P.J. Sebastiana, J. Ramiı´rezc, Int. J. Hydrogen Energy 32 (2007) 3147. [3] Y. Takasu, N. Ohashi, X.G. Zhang, Y. Murakami, H. Minagawa, K. Yahikozawa, Electrochim. Acta 4 (1996) 2595. [4] A.A. Tseng, Nanofabrication: Fundamentals and Applications, World Scientific, Singapore, 2008. [5] B. Bhushan, Springer Handbook of Nanotechnology, Springer, Germany, 2004. [6] H. Zhang, R. Jin, C.A. Mirkin, Nano Lett. 4 (2004) 1493. [7] B.W. Maynor, Y. Li, J. Liu, Langmuir 17 (2001) 2575. [8] L.A. Porter Jr., H.C. Choi, J.M. Schmeltzer, A.E. Ribble, L.C.C. Elliott, J.M. Buriak, Nano Lett. 2 (2002) 1369. [9] H.S. Shin, H.J. Yang, Y.M. Jung, S.B. Kim, Vibrat. Spectrosc. 29 (2002) 79. [10] P.S. Hale, P. Kappen, N. Brack, W. Prissanaroon, P.J. Pigram, J. Liesegang, Appl. Surf. Sci. 252 (2006) 2217. [11] Y. Zhang, S. Maupai, P. Schmuki, Surf. Sci. Lett. 551 (2004) L33. [12] D. Fujita, K. Sagisaka, Sci. Technol. Adv. Mater. 9 (2008) 013003. [13] J.P. Rabe, S. Buchholz, Appl. Phys. Lett. 58 (1991) 7. [14] P. Mesquida, A. Stemmer, Microelectron. Eng. 61 (2002) 671. [15] R.M. Nyffenegger, R.M. Penner, Chem. Rev. 97 (1997) 1195. [16] N. Kubo, T. Homma, Y. Hondo, T. Osaka, Electrochim. Acta 51 (2005) 834. [17] E. Balaur, T. Djenizian, R. Boukherroub, J.N. Chazalviel, F. Ozanam, P. Schmuki, Electrochem. Commun. 6 (2004) 153. [18] C.G. Choi, C. Kee, H. Schift, Curr. Appl. Phys. 6S1 (2006) e8–e11. [19] H. Li, J.M. Biser, J.T. Perkins, S. Dutta, R.P. Vinci, H.M. Chan, J. Appl. Phys. 103 (2008) 024315. [20] P. Schmuki, L.E. Erickson, Phys. Rev. Lett. 85 (2000) 2985. [21] V. Parekh, E. Chunsheng, D. Smith, A. Ruiz, J.C. Wolfe, P. Ruchhoeft, E. Svedberg, S. Khizroev, D. Litvinov, Nanotechnology 17 (2006) 2079. [22] K. Korda´s, J. Remes, S. Leppa¨vuori, L. Na´nai, Appl. Surf. Sci. 178 (2001) 93. [23] C. Scheck, P. Evans, R. Schad, Appl. Phys. Lett. 86 (2005) 133108. [24] S.H. Kim, K.D. Lee, J.Y. Kim, M.K. Kwon, S.J. Park, Nanotechnology 18 (2007) 055306. [25] H. Sugimura, O. Takai, N. Nakagiri, J. Electroanal. Chem. 473 (1999) 230. [26] L.A. Porter Jr., A.E. Ribbe, J.M. Buriak, Nano Lett. 3 (2003) 1043. [27] Y. Zhang, E. Balaur, P. Schmuki, Electrochim. Acta 51 (2006) 3674. [28] H. El-Sayed, M.T. Greiner, P. Kruse, Appl. Surf. Sci. 253 (2007) 8962. [29] L. Santinacci, T. Djenizian, H. Hildebrand, S. Ecoffey, H. Mokdad, T. Campanella, P. Schmuki, Electrochim. Acta 48 (2003) 3123. [30] S.Y. Chou, P.R. Krauss, P. Renstrom, J. Appl. Phys. Lett. 67 (1995) 21. [31] H.J.H. Chen, L.C. Chen, C. Lien, S.R. Chen, Y.L. Ho, Microelect. Eng. 85 (2008) 1561. [32] A.D. Taylor, B.D. Lucas, L.J. Guob, L.T. Thompsona, J. Power Sources 171 (2007) 218. [33] J.H. Lee, K.Y. Yang, S.H. Hong, H. Lee, K.W. Choi, Microelectron. Eng. 85 (2008) 710. [34] M. Fukuhara, J. Mizuno, M. Saito, T. Homma, S. Shoji, Trans. Elect. Elect. Eng. 2 (2007) 307.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nano peel-off fabrication pattern with polymer overcoat layer on glassy carbon electrodes for Pt electrodeposition Akira Kishi, Minoru Umeda * Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 March 2009 Received in revised form 23 June 2009 Accepted 30 June 2009 Available online 4 July 2009
This article describes a new technique for fabricating an electrocatalyst model in which the particle size and interparticle distance are controlled independently. We designed a uniform insulating polymer layer as a mask on an electroconductive glassy carbon substrate and then peeled off a part of the layer in nanosized dots by scratching the overcoat layer using an atomic force microscope (AFM) cantilever. Pt particles electrodeposited only on the peeled off area of the glassy carbon. To peel-off a small area on the glassy carbon, a 29 2 nm thick insulating polymer overcoat layer and a cantilever operating area of 10 nm 10 nm were used, and the smallest peel-off area obtained was 30 nm 30 nm. Thereafter, we performed the peel-off procedure on the polymer overcoat layer of the glassy carbon substrate having a cantilever operating area of 80 nm 80 nm. Pt deposition of 100–150 nm in diameter was successfully achieved by adjusting the interparticle distance. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.07. b Keywords: Nano peel-off pattern fabrication Pt electrodeposition Glassy carbon electrode Polymer overcoat layer and atomic force microscopy
1. Introduction Metallic nano-sized dots are widely used in practical applications of electrocatalysts. Almost all of the electrocatalysts have a structure that loads nano-sized and zero-dimension particles on an electroconductive support. The particle size and interparticle distance are believed to play an important role in functional electrode catalysts [1–3]. A smaller particle size yields a larger surface area, which will supply a larger number of reaction sites. However, to the best of our knowledge there is no research report in the field of electrocatalysts, in which particle size and interparticle distance are independently controlled. With regard to the field of electrocatalysts, it is worthwhile to clarify the relationship between particle size and interparticle distance. To realize a typical electrocatalyst model in which the particle size and interparticle distance are controlled independently, a metallic nano fabrication on an electroconductive substrate seems to be an effective approach [4,5]. Then, we overview of the recent reports on metal nano fabrication. There are two metal nano fabrication techniques: maskless fabrication [6–16] and mask fabrication [17–34]. Maskless fabrication uses a dip pen [6–8], microcontact printing [9,10], metal deposition onto a sample using a charged surface potential [11–15], and an indent method [16].
* Corresponding author. Tel.: +81 258 47 9323; fax: +81 258 47 9300. E-mail address: [email protected] (M. Umeda). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.06.125
For mask fabrication, part of the mask prepared on a substrate is removed in lines by an electron beam [17–19], ion-beam [20–22], chemical etching [23–25], a atomic force microscope (AFM) cantilever [26–29], mold [30–34], etc., where the metallic lines are selectively deposited on the substrate in places where the mask was removed. However, till date, metallic nano fabrication studies have focused on electrode-pattern formation for semiconductor devices [8,10,12,14,15,17–20,23,25–31,34]. Therefore, the metallic nano fabrication has used a technique that deposits a single dimension line of metal on a semiconductor Si substrate, and there are no research reports on the deposition of zero-dimensional metal dots on an electroconductive substrate. To fabricate the electrocatalyst model, we focus our attention on a mask fabrication method that deposits metallic nano particles on the substrate. This method can deposit metallic particles that are of the same size as the area of the mask removed from arbitrary positions on the electroconductive substrate. One noteworthy mask fabrication method employs a line scratch technique that removes the mask using an AFM cantilever [26,27,29]. We intend to develop this line scratch method into a dot peel-off method. This new mask fabrication method is demonstrated in Fig. 1. In this work, an insulating polymer overcoat layer was first prepared by varying its thickness on an electroconductive glassy carbon substrate. Thereafter, the polymer overcoat layer is peeled off by a scratch technique using an AFM cantilever. In this process, the peel-off width and inter peel-off distance of the polymer
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Fig. 1. Schematic of the nano-patterning process: (a) a polymer over coat layer is formed on glassy carbon; (b) peel-off patterning of polymer overcoat layer using a cantilever of AFM; and (c) Pt electrodeposition.
overcoat layer are controlled by changing the operating width and inter peel-off distance of the cantilever. Then, Pt particles are electrodeposited only on the peeled off area of the glassy carbon. Using this procedure, the Pt electrocatalyst model for fuel cells is fabricated successfully.
2. Experimental 2.1. Preparation of glassy carbon with polymer overcoat A glassy carbon plate was purchased from Tokai Carbon Co., Ltd. (GC-20SS). The glassy carbon plate was cut into 1 cm2 pieces. One side of the glassy carbon plate was polished using a buff (Refinetec Co., Ltd., No. 55–308) and alumina (Union Carbide, 0.05 mm in diameter) suspension. Thereafter, the polished glassy carbon was rinsed using Milli-Q water and then dried. The roughness of the polished surface was measured using an AFM (Shimadzu, SPM-9500 J3), and the arithmetic average roughness (Ra) was around 2.2 nm. Next, the polished glassy carbon was immersed in a perfluorinated resin solution (Asahi glass Co., Ltd., Cytop, CTX-109, 2– 4.5 wt.%), and it was pulled up at a constant speed of 0.7 cm min 1 using a motorized micromanipulator (Narishige Co., Ltd., MM-80). After this dip coating procedure, the glassy carbon was dried at
150 8C in an oven chamber for 1 h to form a 20–115 nm thick polymer overcoat layer. 2.2. Micro and nano peel-off of the polymer overcoat The polymer overcoat layer was scratched by an AFM equipped with a Si cantilever (Nano World, PPP-NCHR) in a contact mode to peel-off the polymer overcoat layer. The scanning speed, loading force and number of repeated scanning of the cantilever were 2–1200 nm s 1, 0.6–4.3 mN and 2–5 times, respectively. After scratching, the sample was immersed for 5 min in Milli-Q water to brush off the polymer rubbish by ultrasonic cleaning. 2.3. Pt electrodeposition on the nano peel-off substrate The scratched sample was used as a working electrode for Pt electrodeposition. Ag/Ag2SO4 and Pt wires were used as reference and counter electrodes, respectively. Using a potentiostat (ALS, Model 802B), Pt electrodeposition was carried out five times in a solution mixture of 5 mmol dm 3 H2PtCl6 + 40 mmol dm 3 HCl for 0.1 s at an electrode potential of 0.6 V vs. Ag/Ag2SO4 with an interval of 5 s at 20 8C. Then, the sample was thoroughly rinsed in Milli-Q water and dried.
Fig. 2. Glassy carbon substrate having a polymer overcoat layer with an AFM-based 10 mm width scratch. (a) AFM topography; (b) a cross-sectional profile of the dashed line of (a); (c and d) EDS elemental mappings of F and C of (a).
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The sample surface was characterized by AFM and a scanning electron microscope (SEM) (JEOL, JSM-6060A) coupled with an energy dispersive X-ray spectroscopy (EDS) (JEOL, JED-2300). 3. Results and discussion 3.1. Micro-scale peel-off of the overcoat layer prepared on glassy carbon To realize the concept shown in Fig. 1, an insulating polymer overcoat layer about 100 nm thick was first formed on the glassy carbon substrate. The peel-off conditions of the overcoat layer were an operating area of 10.0 mm 10.0 mm, a scanning speed of 120 mm s 1, a loading force of 4.3 mN, and five repeated scannings of the cantilever. The surface topography of the resulting sample measured by AFM is shown in Fig. 2(a). The bright and dark parts of the figure correspond to the ridge and trench of the sample, respectively. A cross-sectional view of the area marked with a dotted line in Fig. 2(a) is shown in (b). From Fig. 2(b), it appears that the overcoat layer was removed from the cantilever operating area. Fig. 2(c and d) shows EDS elemental mapping of F and C of the same sample. In Fig. 2(c), an infinitesimal F-based signal originating in the perfluorinated polymer layer is observed in the cantilever operating area. Moreover, from Fig. 2(d), the same area of the glassy carbon is considered to be exposed, because the C signal from the glassy carbon is very high. These two results indicated that the insulating perfluorinated resin was perfectly peeled off, and the electroconductive glassy carbon was exposed in the cantilever operating area. Therefore the step height shown in Fig. 2(b) is believed to be equal to the thickness of the polymer overcoat layer. As a result, the thickness of the polymer overcoat layer is estimated to be 106 6 nm. 3.2. Relationship between a nano scale cantilever operating area and the peeled off area The cantilever operating area was downsized to a nano scale, and a relationship between an operating area and a peeled off area was investigated. In this experiment, a 41 4 nm thick insulating layer was formed on the glassy carbon. The thickness was obtained in a manner similar to that described for Fig. 1. Fig. 3(a) shows an AFM image of the sample after a 500 nm 500 nm cantilever operation. Fig. 3(b) shows a cross-sectional view of the area indicated by dotted line in Fig. 3(a). From Fig. 3(b), the dark 600 nm 600 nm area in Fig. 3(a) is known to be flat, and its average depth is 42 1 nm. On comparing the thickness of the polymer overcoat layer and the average depth of the scratched part, it was found that the insulating polymer overcoat layer was peeled off completely and the electroconductive glassy carbon appeared. Experiments similar to those shown in Fig. 3 were performed by changing the film thickness and the cantilever operating area. In Fig. 4, the results are shown in terms of a relationship between the cantilever operating width and peel-off width of the polymer overcoat layer as a function of the overcoat layer thickness. The dotted lines in the figure indicate a slope of 1.0. The peel-off width becomes narrower as the cantilever operating width is reduced. When the cantilever operating width is constant, the peel-off width of a 106 6 nm thick overcoat layer is greater than that of 29 2 and 41 4 nm thick layers. This can be explained by the cantilever operating procedure for a complete peel-off, which requires a number of repetitive scratching for a thicker overcoat layer [29]. Only for the 29 2 nm thick samples, even when the cantilever is operated at less than 100 nm, the peel-off width becomes proportionally narrow as the operating cantilever width is reduced. When
Fig. 3. (a) AFM topography of a glassy carbon substrate with a polymer overcoat layer after the cantilever peel-off. The thickness of the polymer overcoat layer was 41 4 nm and the operating area was 500 nm 500 nm. (b) Cross-sectional profile of the dashed line of (a).
the cantilever operation width is 10 nm, the narrowest peel-off width of 30 nm can be achieved. 3.3. Pt electrodeposition on nano peeled off patterns on the glassy carbon The cantilever scratches were conducted for a 41 4 nm thick insulating layer formed on the glassy carbon. Fig. 5(a) shows an AFM image of the peel-off sample, which has nine scratches prepared by an operating area of 100 nm 100 nm and an inter peel-off distance of 1.5 mm. In the figure, nine dark spots are observed with an interdistance of 1.5 mm. Fig. 5(b) shows a cross-sectional view of
Fig. 4. The operating width vs. peel-off width as a function of the polymer overcoat layer thickness. *: 106 6; : 41 4; : 29 2 nm.
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Fig. 5. (a) AFM image of the peel-off sample which has nine scratches prepared with an operating area of 100 nm 100 nm and an interdistance of 1.5 mm; (b) a crosssectional view of the dotted line of (a); (c) SEM image and (d) EDS elemental mapping of Pt after conducting Pt electrodeposition on the sample in (a).
the dotted line of Fig. 5(a). This figure shows that the depths of the scratched area were 42 1 nm. Clearly, the electroconductive glassy carbon is exposed, because the scratch depth and thickness of the polymer overcoat layer were almost the same.
Fig. 5(c and d) shows an SEM image and an EDS elemental mapping of the Pt after a Pt electrodeposition on the sample in Fig. 5(a). Comparing Fig. 5(a and c) shows that something is deposited in the peeled off area. This deposition is not a single
Fig. 6. (a) AFM image of the peel-off sample which has four scratches prepared with an operating area of 80 nm 80 nm and an interdistance of 500 nm; (b) a cross-sectional view of the dotted line of (a); (c) SEM image and (d) EDS elemental mapping of Pt after electrodeposition on the sample in (a).
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particle, but an aggregation of 50–100 nm thick particles. The EDS elemental mapping, as shown in Fig. 5(d), indicates that Pt electrodeposition took place. In addition, an infinitesimal Cl signal from H2PtCl6 for electrodeposition is observed from the EDS analysis. As a result, Pt particles were electrodeposited only on the exposed parts of the glassy carbon substrate. Next, the scratch was downsized using the sample having a 29 2 nm thick insulating layer on the glassy carbon. Fig. 6(a) shows an AFM image of the peeled off result, which has four scratches prepared by operating area of 80 nm 80 nm and an interdistance of 500 nm. In the figure, four dark spots are observed with an interval of 500 nm. Fig. 6(b) shows a cross-sectional view of the dotted line of Fig. 6(a), indicating that the scratch depths were 31 2 nm and the peel-off width was 100 nm. It is apparent that the electroconductive glassy carbon was exposed, because the scratch depth and thickness of the polymer overcoat layer are almost the same. Therefore, the sample in Fig. 6(a) successfully downsized the peel-off width and the interdistance, compared with the sample in Fig. 5(a). This achievement in downsizing is attributed to the fact that the thickness of the polymer overcoat layer of the sample in Fig. 6(a) was less than that of the sample in Fig. 5(a). Fig. 6(c and d) shows an SEM image and an EDS elemental mapping of Pt after Pt electrodeposition on the sample of Fig. 6(a). When we compare Fig. 6(a and c), some deposition is seen in the peeled off parts. In this experiment, the EDS elemental mapping, as shown in Fig. 6(d), indicates that Pt electrodeposition took place and no Cl-based signal was observed from the EDS analysis. As a result, Pt particles were electrodeposited only on the exposed parts of the glassy carbon substrate. Next, Pt electroplating was attempted for the 30 nm peeled off sample, resulted in no Pt deposition. Although the glassy carbon substrate is known to be exposed by the peel-off from the result shown in Fig. 6, the reason can be explained that the electrolytic solution is hard to contact with the electrode through the small holes. In conclusion, we succeeded in fabricating the Pt clusters, 100– 150 nm in diameter, with an interparticle distance of 500 nm, within limited areas of the glassy carbon substrate using a newly developed mask fabrication technique. In the future, we have a plan to do experiments on 30 nm width Pt by preparing a thinner insulating layer on the electrode substrate. 4. Conclusions We developed a new mask fabrication technique for fabricating nano-sized Pt particles at arbitrary positions on an electroconductive glassy carbon substrate. Portions of a polymer overcoat layer were peeled off by an AFM cantilever, and Pt particles were electrodeposited only on the peeled off parts of the glassy carbon. First, the peel-off procedure of the polymer overcoat layer prepared on the glassy carbon substrate was investigated. The cantilever operation perfectly peeled off the insulating perfluorinated resin and exposed the electroconductive glassy carbon. This enabled the estimation of the thickness of the polymer overcoat layer.
Second, we investigated the relationship between the cantilever operating width and the peel-off width of the overcoat layer as a function of its thickness. The resulting relationship showed that 30 nm was the narrowest width that could be peeled off, and it was achieved when the polymer overcoat layer was 29 2 nm thick and the cantilever operating width was 10 nm. Finally, for fabricating a Pt-dot-pattern electrode, a 100 nm wide peel-off pattern, with a 500 nm inter peel-off distance for a 29 2 nm thick overcoat layer was prepared. After electrodepositing Pt on the sample, Pt particles were deposited only on the parts of the substrate where the overcoat layer was removed. In conclusion, we successfully fabricated a Pt electrocatalyst model using a newly developed mask fabrication technique. Acknowledgement Part of this work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] H. Yano, J. Inukai, H. Uchida, M. Watanabe, P.K. Babu, T. Kobayashi, J.H. Chung, E. Oldfield, A. Wieckowski, Phys. Chem. Chem. Phys. 42 (2006) 4932. [2] L.C. Ordo´n˜ez, P. Roqueroc, P.J. Sebastiana, J. Ramiı´rezc, Int. J. Hydrogen Energy 32 (2007) 3147. [3] Y. Takasu, N. Ohashi, X.G. Zhang, Y. Murakami, H. Minagawa, K. Yahikozawa, Electrochim. Acta 4 (1996) 2595. [4] A.A. Tseng, Nanofabrication: Fundamentals and Applications, World Scientific, Singapore, 2008. [5] B. Bhushan, Springer Handbook of Nanotechnology, Springer, Germany, 2004. [6] H. Zhang, R. Jin, C.A. Mirkin, Nano Lett. 4 (2004) 1493. [7] B.W. Maynor, Y. Li, J. Liu, Langmuir 17 (2001) 2575. [8] L.A. Porter Jr., H.C. Choi, J.M. Schmeltzer, A.E. Ribble, L.C.C. Elliott, J.M. Buriak, Nano Lett. 2 (2002) 1369. [9] H.S. Shin, H.J. Yang, Y.M. Jung, S.B. Kim, Vibrat. Spectrosc. 29 (2002) 79. [10] P.S. Hale, P. Kappen, N. Brack, W. Prissanaroon, P.J. Pigram, J. Liesegang, Appl. Surf. Sci. 252 (2006) 2217. [11] Y. Zhang, S. Maupai, P. Schmuki, Surf. Sci. Lett. 551 (2004) L33. [12] D. Fujita, K. Sagisaka, Sci. Technol. Adv. Mater. 9 (2008) 013003. [13] J.P. Rabe, S. Buchholz, Appl. Phys. Lett. 58 (1991) 7. [14] P. Mesquida, A. Stemmer, Microelectron. Eng. 61 (2002) 671. [15] R.M. Nyffenegger, R.M. Penner, Chem. Rev. 97 (1997) 1195. [16] N. Kubo, T. Homma, Y. Hondo, T. Osaka, Electrochim. Acta 51 (2005) 834. [17] E. Balaur, T. Djenizian, R. Boukherroub, J.N. Chazalviel, F. Ozanam, P. Schmuki, Electrochem. Commun. 6 (2004) 153. [18] C.G. Choi, C. Kee, H. Schift, Curr. Appl. Phys. 6S1 (2006) e8–e11. [19] H. Li, J.M. Biser, J.T. Perkins, S. Dutta, R.P. Vinci, H.M. Chan, J. Appl. Phys. 103 (2008) 024315. [20] P. Schmuki, L.E. Erickson, Phys. Rev. Lett. 85 (2000) 2985. [21] V. Parekh, E. Chunsheng, D. Smith, A. Ruiz, J.C. Wolfe, P. Ruchhoeft, E. Svedberg, S. Khizroev, D. Litvinov, Nanotechnology 17 (2006) 2079. [22] K. Korda´s, J. Remes, S. Leppa¨vuori, L. Na´nai, Appl. Surf. Sci. 178 (2001) 93. [23] C. Scheck, P. Evans, R. Schad, Appl. Phys. Lett. 86 (2005) 133108. [24] S.H. Kim, K.D. Lee, J.Y. Kim, M.K. Kwon, S.J. Park, Nanotechnology 18 (2007) 055306. [25] H. Sugimura, O. Takai, N. Nakagiri, J. Electroanal. Chem. 473 (1999) 230. [26] L.A. Porter Jr., A.E. Ribbe, J.M. Buriak, Nano Lett. 3 (2003) 1043. [27] Y. Zhang, E. Balaur, P. Schmuki, Electrochim. Acta 51 (2006) 3674. [28] H. El-Sayed, M.T. Greiner, P. Kruse, Appl. Surf. Sci. 253 (2007) 8962. [29] L. Santinacci, T. Djenizian, H. Hildebrand, S. Ecoffey, H. Mokdad, T. Campanella, P. Schmuki, Electrochim. Acta 48 (2003) 3123. [30] S.Y. Chou, P.R. Krauss, P. Renstrom, J. Appl. Phys. Lett. 67 (1995) 21. [31] H.J.H. Chen, L.C. Chen, C. Lien, S.R. Chen, Y.L. Ho, Microelect. Eng. 85 (2008) 1561. [32] A.D. Taylor, B.D. Lucas, L.J. Guob, L.T. Thompsona, J. Power Sources 171 (2007) 218. [33] J.H. Lee, K.Y. Yang, S.H. Hong, H. Lee, K.W. Choi, Microelectron. Eng. 85 (2008) 710. [34] M. Fukuhara, J. Mizuno, M. Saito, T. Homma, S. Shoji, Trans. Elect. Elect. Eng. 2 (2007) 307.