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Electrochemical deposition of Pd nanoparticles on indium-tin oxide electrodes and their catalytic properties for formic acid oxidation
Electrochemistry Communications 12 (2010) 1442–1445
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Electrochemical deposition of Pd nanoparticles on indium-tin oxide electrodes and their catalytic properties for formic acid oxidation Byung-Kwon Kim, Daeha Seo, Ji Young Lee, Hyunjoon Song, Juhyoun Kwak ⁎ Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 July 2010 Received in revised form 3 August 2010 Accepted 3 August 2010 Available online 10 August 2010 Keywords: Pd nanoparticles Electrochemical deposition Formic acid Cyclic voltammetry Catalyst
a b s t r a c t Pd nanoparticles (NPs) were directly deposited on indium-tin oxide (ITO) electrodes by cyclic voltammetry (CV) in a bulk Pd2+ solution and the size of the Pd (NPs) was evaluated by SEM. The electrochemical deposition conditions of the Pd NPs were varied according to a scan rate. As the scan rate was decreased, the size of the Pd NPs increased, but the formic acid catalytic property was weakened. With regard to cycle number, with increased cycling, the size of the Pd NPs increased but the formic acid catalytic property decreased. As the conditions of electrochemical deposition were varied, the particle size and catalytic activity for formic acid were also changed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Pd is a very important and rare transition metal that has good catalytic activity and its catalytic activity have been widely studied by many researchers [1–3]. Pd has strong potential for application in hydrogen storage materials [4], chemical sensors [5], and catalysts for fuel cell [3,6,7]. Numerous studies have also focused on the use of thin Pd films in variable metal electrodes [1,8–13], as well as the application of Pd as an alloy with other metals [4,14]. Research is still needed, however, on the fabrication of Pd NPs in order to increase its catalytic properties and applications [15,16]. ITO electrode has been widely used in various areas such as a solar cell, an organic-light-emitting-diode, and a photovoltaic device because of their high optical transparency and good conductivity. Recently, a few papers about NPs formation using direct-electrodeposition on ITO electrode have been reported. Compton et al. reported electrodeposition of Au NPs on ITO electrodes for application to the detection of arsenic(III) [17]. Wang et al. reported one-step electrodeposition of Pt nanoflowers on ITO electrode and their catalytic activity for methanol oxidation [7]. Research is still needed about NPs formation on ITO electrode. Much attention has been focused on solution-based synthesis and the catalytic activities of bimetallic NPs with Pd and other noble metals. These kinds of bimetallic NPs show very good catalytic activity
and resistivity to poisoning [18–20]. Solution-based synthesis of Pd NPs, however, involves complex and also requires a polymer stabilizer for dispersion of Pd NPs. In addition, synthesized Pd NPs require a binding step for the NPs and electrodes. However, electrochemical deposition techniques have many advantages compared to solutionbased NP synthesis [7,17,21]. The process can be simply controlled by adjusting the applied current, voltage, and time. Therefore, control over the electrochemical deposition conditions of NPs is also very feasible. In addition, since the electrochemical deposition is implemented on an electrode, the electrochemically deposited NPs do not require an additional processing step of adding binder to the electrodes. Herein, we studied electrochemical deposition of Pd NPs and their catalytic activity for formic acid. Pd2+ was electrochemically deposited on ITO electrodes. We used cyclic voltammetry (CV) for electrochemical deposition of Pd NPs and varied the scan rate and cycle number. It was found that at a low scan rate, the size and amount of deposited NPs respectively increased. On the other hand, the formic acid oxidation ability was lower than that obtained with a high scan rate. With respect to the cycle number, as the cycle number was increased, the size and amount of deposited Pd NPs increased but the formic acid oxidation ability was lower than that obtained with a small cycle number. 2. Experiment details
⁎ Corresponding author. Tel.: +82 42 350 2833; fax: +82 42 350 2810. E-mail address: [email protected] (J. Kwak). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.08.004
PdCl2, H2SO4, HCl, and formic acid were purchased from SigmaAldrich. ITO electrodes were supplied from Geomatec (Yokohama, Japan). The electrochemical experiment was performed using a CHI 900B electrochemical analyzer (CH instrument, USA). A standard
B.-K. Kim et al. / Electrochemistry Communications 12 (2010) 1442–1445
three-electrode setup was used with an ITO working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The area of the working electrode was 0.28 cm2. Pd NPs were electrodeposited on ITO electrodes by CV with a 0.1 M H2SO4 + 0.1 mM PdCl2 + 0.2 mM HCl solution. Current density ratio by scan rate was compiled statistics from independent three data. Formic acid oxidation was measured by CV with a 0.1 M H2SO4 + 0.2 M formic acid solution. The formic acid solutions were deaerated with Ar bubbling. The average size of the Pd NPs was estimated by counting 200 particles in SEM images. 3. Results and discussion Pd NPs were electrochemically deposited by CV according to the scan rate. The scan rate was varied as 1, 5, and 10 mV/s (Fig. 1(a)). In all cases, reduction current of Pd2+ was shown at 0.14 V. The current density at 0.14 V reflects the formation from Pd2+ to Pd NPs on ITO electrodes. This is a substantially low reduction potential compared to other metal electrodes, and is attributable to the low catalytic activity of ITO electrodes [8,13,22]. The low catalytic activity of ITO electrodes hinders nucleation of Pd2+. The current density at 0.14 V was increased by increasing the scan rate. The equation giving the peak current (iP) in a totally irreversible system can explain the increase of the current density at 0.14 V: [23] 5 1=2 T 1 = 2 1=2 ip = 2:99 × 10 α AC0 D0 v
ð1Þ
where α is a transfer coefficient, A is area, C*0 is the bulk concentration of oxidant, D0 is the diffusion coefficient of oxidant, and v is the scan rate. According to Eq. (1), ip is proportional to v1/2, and the results of Fig. 1(a) coincide with Eq. (1). The current density at 0.14 V and scan rates of 1, 5, and 10 mV/s was 29.2 ± 1.6, 62.6 ± 4.5, and 94.1 ± 5.8 μA/ pffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi cm2, respectively. The current density ratio is 1: 4:6: 10:4, which is p ffiffiffiffiffi ffi p ffiffiffi similar to the v1/2 ratio 1: 5: 10. Therefore, the reduction current density of Pd2+ at 0.14 V agrees well with Eq. (1). Fig. 2 shows SEM images of deposited Pd NPs. The average size of the Pd NPs at scan rates of 1, 5, and 10 mV/s were 223 ± 38, 179 ± 31, and 97 ± 27 nm, respectively (Fig 2(a)). As the scan rate was increased, the particle size decreased. In the case of low scan rate, Pd2+ has sufficient time to approach the seeds of Pd NPs and particle growth occurred on the Pd NP surface. Because of the low catalytic activity of ITO electrodes, particle growth on the Pd NP surface was preferred over nucleation on the surface of the ITO electrodes. Therefore, at a low scan rate, the particle size of the Pd NPs increased (Fig. 2(a)). The catalytic activity of the electrochemically deposited Pd NPs under different scan rates was measured. Fig. 3 shows the CVs of formic acid oxidation by the electrochemically deposited Pd NPs. As
1443
shown in Fig. 3, the anodic peak around 0.05 V reflects oxidation of formic acid catalyzed by Pd NPs and the peak at 0.4 V in the reverse scan indicates reduction of Pd oxide [1,15,19]. After removing oxide layer of the Pd NPs, the surface of the Pd NPs was cleaned and formic acid oxidation occurred again around 0.05 V. The current density at 0.05 V was the highest with a scan rate of 10 mV/s (Fig. 3(a-iii)). At a low scan rate, the catalytic activity for formic acid was also low. While the particle size of Pd NPs was increased readily by decreasing the scan rate, the catalytic activity of the electrochemically deposited Pd NPs did not increase. Pd NPs were electrochemically deposited by increasing the cycle number of CV to 5 and 10 times while keeping the scan rate fixed at 10 mV/s (Fig. 1(b,c)). When the first scan was completed, reduction currents of Pd2+ were shown at 0.14 V. This reduction current represents the formation of Pd NPs on the surface of the ITO electrodes, and the result corresponds with that obtained in the single cycle experiment (Fig. 1(a-iii)). However, the peak shape and potential from the second to fifth scans were substantially different from those of the first scan. After the first scan, the cathodic current density of Pd2+ was shifted from 0.14 to 0.35 V. This means that the electrochemical deposition conditions of Pd2+ were significantly changed. At the first scan, nucleation and growth of Pd2+ were generated only on the surface of the ITO electrodes. However, as noted earlier, the ITO electrodes have low electrocatalytic activity and low capacitive current. Therefore, reduction of Pd2+ is difficult on ITO electrodes. However, after the first scan, Pd NPs were already formed on the ITO electrodes and the Pd2+ were electrochemically deposited more readily on the surface of the Pd NPs than on the surface of the ITO electrodes. Therefore, the peak at 0.35 V is the reduction peak of Pd2+ on Pd NPs. The result for deposition via 10 cycles, shown in Fig. 1 (c), is similar to that for 5 cycle deposition. The Pd2+ reduction peak of the first scan on the ITO electrodes was shown at 0.14 V but the reduction peaks on the surface of the Pd NPs from the second to tenth cycle were shown around 0.35 V. The current density at 0.35 V was slightly increased due to the increased particle size and number. Fig. 2(b,c) shows SEM images of Pd NPs deposited on ITO electrodes. As shown here, as the cycle number was increased, the size and number of Pd NPs also increased. The average size of Pd NPs at 5 and 10 cycles were 164 ± 31 and 213 ± 48 nm, respectively. We measured the catalytic activity of Pd NPs for formic acid. CVs of formic acid oxidation by Pd NPs are shown in Fig. 3(b,c). The current density at 0.05 V of formic acid was decreased as the number of CV cycles was increased. However, according to the SEM images, the size and number of Pd NPs clearly increased by increasing the cycle number (Fig. 2(b,c)). In spite of the numerous and large Pd NPs, the oxidation current density was lower than that in the case of single cycle deposition (Fig. 3(a-iii)). We believe this indicates that increasing the size of Pd NPs has a negative effect on their catalytic
Fig. 1. CVs for Pd NPs electrochemical deposition on ITO electrodes at scan rate (a-i) 1, (a-ii) 5, and (a-iii, b, c) 10 mV/s. Cycle number is (a) 1, (b) 5, and (c) 10 cycles.
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B.-K. Kim et al. / Electrochemistry Communications 12 (2010) 1442–1445
Fig. 2. SEM images of Pd NPs after electrochemical deposition on ITO electrodes at scan rate (a-i) 1, (a-ii) 5, and (a-iii, b, c) 10 mV/s. Cycle number is (a) 1, (b) 5, and (c) 10 cycles.
activity. In the case of deposition at 10 cycles, the same phenomena were observed. The size and number of Pd NPs were increased (Fig. 2 (c)) but the formic acid oxidation current density decreased (Fig. 3 (c)). 4. Conclusions We have investigated the electrochemical deposition of Pd NPs on ITO electrodes by CV. This electrochemical synthesis of Pd NPs can be simply controlled by the scan rate and cycle number. According to the scan rate and cycle number, the size and amount of deposited Pd NPs varied. At a low scan rate and high cycle number, the size of the Pd NPs
increased. However, despite the increase in particle size, the current density of formic acid oxidation decreased due to a reduction of the catalytic activity of the Pd NPs. In our experiments, the best results were obtained with a scan rate of 10 mV/s scan rate and 1 cycle deposition. We believe that the findings of this study will benefit research on electrochemical deposition of Pd NPs and Pd catalysts for formic acid oxidation. Acknowledgments This work was supported by the Nano/Bio Science & Technology Program (2010-0008213) of the Ministry of Education, Science and
Fig. 3. CVs for the catalytic activity of Pd NPs. The Pd NPs were electrochemically deposited at scan rate (a-i) 1, (a-ii) 5, and (a-iii, b, c) 10 mV/s. Cycle number is (a) 1, (b) 5, and (c) 10 cycles.
B.-K. Kim et al. / Electrochemistry Communications 12 (2010) 1442–1445
Technology, and the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (2010-0001951). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11375. R. Wang, S. Liao, S. Ji, J. Power Sources 180 (2008) 205. V. Raghuveer, A. Manthiram, A.J. Bard, J. Phys. Chem. B 109 (2005) 22909. M. Grdeń, A. Piaścik, Z. Koczorowski, A. Czerwiński, J. Electroanal. Chem. 532 (2002) 35. J. DAS, H. Kim, K. Jo, K.H. Park, S. Jon, K. Lee, H. Yang, Chem. Commun. (2009) 6394. X. Ji, K.T. Lee, R. Holden, L. Zhang, J. Zhang, G.A. Botton, M. Couillard, L.F. Nazar, Nat. Chem. 2 (2010) 286. H. Zhang, W. Zhou, Y. Du, P. Yang, C. Wang, Electrochem. Commun. 12 (2010) 882. A.L.N. Pinheiro, M.S. Zei, M.F. Luo, G. Ertl, Surf. Sci. 600 (2006) 641. H. Duncan, A. Lasia, Electrochim. Acta 52 (2007) 6195.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
1445
R. Hoyer, L.A. Kibler, D.M. Kolb, Surf. Sci. 562 (2004) 275. A.M. El-Aziz, L.A. Kiber, D.M. Kolb, Electrochem. Commun. 4 (2002) 535. H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102 (1998) 4366. M.E. Quayum, Ye. Shen, K. Uosaki, J. Electroanal. Chem. 520 (2002) 126. A.N. Correia, L.H. Mascaro, S.A.S. Machado, L.A. Avaca, Electrochim. Acta 42 (1997) 493. R.K. Pandey, V. Lakshminarayanan, J. Phys. Chem. C 113 (2009) 21596. W. Pan, X. Zhang, H. Ma, J. Zhang, J. Phys. Chem. C 112 (2008) 2456. X. DAI, R.G. Compton, Anal. Sci. 22 (2006) 567. V. Mazumder, S. Sun, J. Am. Chem. Soc. 131 (2009) 4588. S.E. Habas, H. Lee, V. Radmilovic, G.A. Somorjai, P. Yang, Nat. Mater. 6 (2007) 692. H. Lee, S.E. Habas, G.A. Somorjai, P. Yang, J. Am. Chem. Soc. 130 (2008) 5406. F. Cheng, H. Wang, Z. Sun, M. Ning, Z. Cai, M. Zhang, Electrochem. Commun. 10 (2008) 798. M.A. Aziz, S. Patra, H. Yang, Chem. Commun. (2008) 4607. A.J. Bard, L.R. Faulkner, Electrochemical Methods : Fundamentals and Applications, 2nd ed.Wiley, NY, 2001.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Electrochemical deposition of Pd nanoparticles on indium-tin oxide electrodes and their catalytic properties for formic acid oxidation Byung-Kwon Kim, Daeha Seo, Ji Young Lee, Hyunjoon Song, Juhyoun Kwak ⁎ Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 July 2010 Received in revised form 3 August 2010 Accepted 3 August 2010 Available online 10 August 2010 Keywords: Pd nanoparticles Electrochemical deposition Formic acid Cyclic voltammetry Catalyst
a b s t r a c t Pd nanoparticles (NPs) were directly deposited on indium-tin oxide (ITO) electrodes by cyclic voltammetry (CV) in a bulk Pd2+ solution and the size of the Pd (NPs) was evaluated by SEM. The electrochemical deposition conditions of the Pd NPs were varied according to a scan rate. As the scan rate was decreased, the size of the Pd NPs increased, but the formic acid catalytic property was weakened. With regard to cycle number, with increased cycling, the size of the Pd NPs increased but the formic acid catalytic property decreased. As the conditions of electrochemical deposition were varied, the particle size and catalytic activity for formic acid were also changed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Pd is a very important and rare transition metal that has good catalytic activity and its catalytic activity have been widely studied by many researchers [1–3]. Pd has strong potential for application in hydrogen storage materials [4], chemical sensors [5], and catalysts for fuel cell [3,6,7]. Numerous studies have also focused on the use of thin Pd films in variable metal electrodes [1,8–13], as well as the application of Pd as an alloy with other metals [4,14]. Research is still needed, however, on the fabrication of Pd NPs in order to increase its catalytic properties and applications [15,16]. ITO electrode has been widely used in various areas such as a solar cell, an organic-light-emitting-diode, and a photovoltaic device because of their high optical transparency and good conductivity. Recently, a few papers about NPs formation using direct-electrodeposition on ITO electrode have been reported. Compton et al. reported electrodeposition of Au NPs on ITO electrodes for application to the detection of arsenic(III) [17]. Wang et al. reported one-step electrodeposition of Pt nanoflowers on ITO electrode and their catalytic activity for methanol oxidation [7]. Research is still needed about NPs formation on ITO electrode. Much attention has been focused on solution-based synthesis and the catalytic activities of bimetallic NPs with Pd and other noble metals. These kinds of bimetallic NPs show very good catalytic activity
and resistivity to poisoning [18–20]. Solution-based synthesis of Pd NPs, however, involves complex and also requires a polymer stabilizer for dispersion of Pd NPs. In addition, synthesized Pd NPs require a binding step for the NPs and electrodes. However, electrochemical deposition techniques have many advantages compared to solutionbased NP synthesis [7,17,21]. The process can be simply controlled by adjusting the applied current, voltage, and time. Therefore, control over the electrochemical deposition conditions of NPs is also very feasible. In addition, since the electrochemical deposition is implemented on an electrode, the electrochemically deposited NPs do not require an additional processing step of adding binder to the electrodes. Herein, we studied electrochemical deposition of Pd NPs and their catalytic activity for formic acid. Pd2+ was electrochemically deposited on ITO electrodes. We used cyclic voltammetry (CV) for electrochemical deposition of Pd NPs and varied the scan rate and cycle number. It was found that at a low scan rate, the size and amount of deposited NPs respectively increased. On the other hand, the formic acid oxidation ability was lower than that obtained with a high scan rate. With respect to the cycle number, as the cycle number was increased, the size and amount of deposited Pd NPs increased but the formic acid oxidation ability was lower than that obtained with a small cycle number. 2. Experiment details
⁎ Corresponding author. Tel.: +82 42 350 2833; fax: +82 42 350 2810. E-mail address: [email protected] (J. Kwak). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.08.004
PdCl2, H2SO4, HCl, and formic acid were purchased from SigmaAldrich. ITO electrodes were supplied from Geomatec (Yokohama, Japan). The electrochemical experiment was performed using a CHI 900B electrochemical analyzer (CH instrument, USA). A standard
B.-K. Kim et al. / Electrochemistry Communications 12 (2010) 1442–1445
three-electrode setup was used with an ITO working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The area of the working electrode was 0.28 cm2. Pd NPs were electrodeposited on ITO electrodes by CV with a 0.1 M H2SO4 + 0.1 mM PdCl2 + 0.2 mM HCl solution. Current density ratio by scan rate was compiled statistics from independent three data. Formic acid oxidation was measured by CV with a 0.1 M H2SO4 + 0.2 M formic acid solution. The formic acid solutions were deaerated with Ar bubbling. The average size of the Pd NPs was estimated by counting 200 particles in SEM images. 3. Results and discussion Pd NPs were electrochemically deposited by CV according to the scan rate. The scan rate was varied as 1, 5, and 10 mV/s (Fig. 1(a)). In all cases, reduction current of Pd2+ was shown at 0.14 V. The current density at 0.14 V reflects the formation from Pd2+ to Pd NPs on ITO electrodes. This is a substantially low reduction potential compared to other metal electrodes, and is attributable to the low catalytic activity of ITO electrodes [8,13,22]. The low catalytic activity of ITO electrodes hinders nucleation of Pd2+. The current density at 0.14 V was increased by increasing the scan rate. The equation giving the peak current (iP) in a totally irreversible system can explain the increase of the current density at 0.14 V: [23] 5 1=2 T 1 = 2 1=2 ip = 2:99 × 10 α AC0 D0 v
ð1Þ
where α is a transfer coefficient, A is area, C*0 is the bulk concentration of oxidant, D0 is the diffusion coefficient of oxidant, and v is the scan rate. According to Eq. (1), ip is proportional to v1/2, and the results of Fig. 1(a) coincide with Eq. (1). The current density at 0.14 V and scan rates of 1, 5, and 10 mV/s was 29.2 ± 1.6, 62.6 ± 4.5, and 94.1 ± 5.8 μA/ pffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi cm2, respectively. The current density ratio is 1: 4:6: 10:4, which is p ffiffiffiffiffi ffi p ffiffiffi similar to the v1/2 ratio 1: 5: 10. Therefore, the reduction current density of Pd2+ at 0.14 V agrees well with Eq. (1). Fig. 2 shows SEM images of deposited Pd NPs. The average size of the Pd NPs at scan rates of 1, 5, and 10 mV/s were 223 ± 38, 179 ± 31, and 97 ± 27 nm, respectively (Fig 2(a)). As the scan rate was increased, the particle size decreased. In the case of low scan rate, Pd2+ has sufficient time to approach the seeds of Pd NPs and particle growth occurred on the Pd NP surface. Because of the low catalytic activity of ITO electrodes, particle growth on the Pd NP surface was preferred over nucleation on the surface of the ITO electrodes. Therefore, at a low scan rate, the particle size of the Pd NPs increased (Fig. 2(a)). The catalytic activity of the electrochemically deposited Pd NPs under different scan rates was measured. Fig. 3 shows the CVs of formic acid oxidation by the electrochemically deposited Pd NPs. As
1443
shown in Fig. 3, the anodic peak around 0.05 V reflects oxidation of formic acid catalyzed by Pd NPs and the peak at 0.4 V in the reverse scan indicates reduction of Pd oxide [1,15,19]. After removing oxide layer of the Pd NPs, the surface of the Pd NPs was cleaned and formic acid oxidation occurred again around 0.05 V. The current density at 0.05 V was the highest with a scan rate of 10 mV/s (Fig. 3(a-iii)). At a low scan rate, the catalytic activity for formic acid was also low. While the particle size of Pd NPs was increased readily by decreasing the scan rate, the catalytic activity of the electrochemically deposited Pd NPs did not increase. Pd NPs were electrochemically deposited by increasing the cycle number of CV to 5 and 10 times while keeping the scan rate fixed at 10 mV/s (Fig. 1(b,c)). When the first scan was completed, reduction currents of Pd2+ were shown at 0.14 V. This reduction current represents the formation of Pd NPs on the surface of the ITO electrodes, and the result corresponds with that obtained in the single cycle experiment (Fig. 1(a-iii)). However, the peak shape and potential from the second to fifth scans were substantially different from those of the first scan. After the first scan, the cathodic current density of Pd2+ was shifted from 0.14 to 0.35 V. This means that the electrochemical deposition conditions of Pd2+ were significantly changed. At the first scan, nucleation and growth of Pd2+ were generated only on the surface of the ITO electrodes. However, as noted earlier, the ITO electrodes have low electrocatalytic activity and low capacitive current. Therefore, reduction of Pd2+ is difficult on ITO electrodes. However, after the first scan, Pd NPs were already formed on the ITO electrodes and the Pd2+ were electrochemically deposited more readily on the surface of the Pd NPs than on the surface of the ITO electrodes. Therefore, the peak at 0.35 V is the reduction peak of Pd2+ on Pd NPs. The result for deposition via 10 cycles, shown in Fig. 1 (c), is similar to that for 5 cycle deposition. The Pd2+ reduction peak of the first scan on the ITO electrodes was shown at 0.14 V but the reduction peaks on the surface of the Pd NPs from the second to tenth cycle were shown around 0.35 V. The current density at 0.35 V was slightly increased due to the increased particle size and number. Fig. 2(b,c) shows SEM images of Pd NPs deposited on ITO electrodes. As shown here, as the cycle number was increased, the size and number of Pd NPs also increased. The average size of Pd NPs at 5 and 10 cycles were 164 ± 31 and 213 ± 48 nm, respectively. We measured the catalytic activity of Pd NPs for formic acid. CVs of formic acid oxidation by Pd NPs are shown in Fig. 3(b,c). The current density at 0.05 V of formic acid was decreased as the number of CV cycles was increased. However, according to the SEM images, the size and number of Pd NPs clearly increased by increasing the cycle number (Fig. 2(b,c)). In spite of the numerous and large Pd NPs, the oxidation current density was lower than that in the case of single cycle deposition (Fig. 3(a-iii)). We believe this indicates that increasing the size of Pd NPs has a negative effect on their catalytic
Fig. 1. CVs for Pd NPs electrochemical deposition on ITO electrodes at scan rate (a-i) 1, (a-ii) 5, and (a-iii, b, c) 10 mV/s. Cycle number is (a) 1, (b) 5, and (c) 10 cycles.
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B.-K. Kim et al. / Electrochemistry Communications 12 (2010) 1442–1445
Fig. 2. SEM images of Pd NPs after electrochemical deposition on ITO electrodes at scan rate (a-i) 1, (a-ii) 5, and (a-iii, b, c) 10 mV/s. Cycle number is (a) 1, (b) 5, and (c) 10 cycles.
activity. In the case of deposition at 10 cycles, the same phenomena were observed. The size and number of Pd NPs were increased (Fig. 2 (c)) but the formic acid oxidation current density decreased (Fig. 3 (c)). 4. Conclusions We have investigated the electrochemical deposition of Pd NPs on ITO electrodes by CV. This electrochemical synthesis of Pd NPs can be simply controlled by the scan rate and cycle number. According to the scan rate and cycle number, the size and amount of deposited Pd NPs varied. At a low scan rate and high cycle number, the size of the Pd NPs
increased. However, despite the increase in particle size, the current density of formic acid oxidation decreased due to a reduction of the catalytic activity of the Pd NPs. In our experiments, the best results were obtained with a scan rate of 10 mV/s scan rate and 1 cycle deposition. We believe that the findings of this study will benefit research on electrochemical deposition of Pd NPs and Pd catalysts for formic acid oxidation. Acknowledgments This work was supported by the Nano/Bio Science & Technology Program (2010-0008213) of the Ministry of Education, Science and
Fig. 3. CVs for the catalytic activity of Pd NPs. The Pd NPs were electrochemically deposited at scan rate (a-i) 1, (a-ii) 5, and (a-iii, b, c) 10 mV/s. Cycle number is (a) 1, (b) 5, and (c) 10 cycles.
B.-K. Kim et al. / Electrochemistry Communications 12 (2010) 1442–1445
Technology, and the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (2010-0001951). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11375. R. Wang, S. Liao, S. Ji, J. Power Sources 180 (2008) 205. V. Raghuveer, A. Manthiram, A.J. Bard, J. Phys. Chem. B 109 (2005) 22909. M. Grdeń, A. Piaścik, Z. Koczorowski, A. Czerwiński, J. Electroanal. Chem. 532 (2002) 35. J. DAS, H. Kim, K. Jo, K.H. Park, S. Jon, K. Lee, H. Yang, Chem. Commun. (2009) 6394. X. Ji, K.T. Lee, R. Holden, L. Zhang, J. Zhang, G.A. Botton, M. Couillard, L.F. Nazar, Nat. Chem. 2 (2010) 286. H. Zhang, W. Zhou, Y. Du, P. Yang, C. Wang, Electrochem. Commun. 12 (2010) 882. A.L.N. Pinheiro, M.S. Zei, M.F. Luo, G. Ertl, Surf. Sci. 600 (2006) 641. H. Duncan, A. Lasia, Electrochim. Acta 52 (2007) 6195.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
1445
R. Hoyer, L.A. Kibler, D.M. Kolb, Surf. Sci. 562 (2004) 275. A.M. El-Aziz, L.A. Kiber, D.M. Kolb, Electrochem. Commun. 4 (2002) 535. H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102 (1998) 4366. M.E. Quayum, Ye. Shen, K. Uosaki, J. Electroanal. Chem. 520 (2002) 126. A.N. Correia, L.H. Mascaro, S.A.S. Machado, L.A. Avaca, Electrochim. Acta 42 (1997) 493. R.K. Pandey, V. Lakshminarayanan, J. Phys. Chem. C 113 (2009) 21596. W. Pan, X. Zhang, H. Ma, J. Zhang, J. Phys. Chem. C 112 (2008) 2456. X. DAI, R.G. Compton, Anal. Sci. 22 (2006) 567. V. Mazumder, S. Sun, J. Am. Chem. Soc. 131 (2009) 4588. S.E. Habas, H. Lee, V. Radmilovic, G.A. Somorjai, P. Yang, Nat. Mater. 6 (2007) 692. H. Lee, S.E. Habas, G.A. Somorjai, P. Yang, J. Am. Chem. Soc. 130 (2008) 5406. F. Cheng, H. Wang, Z. Sun, M. Ning, Z. Cai, M. Zhang, Electrochem. Commun. 10 (2008) 798. M.A. Aziz, S. Patra, H. Yang, Chem. Commun. (2008) 4607. A.J. Bard, L.R. Faulkner, Electrochemical Methods : Fundamentals and Applications, 2nd ed.Wiley, NY, 2001.