- Email: [email protected]
Fabrication and electrochemical property of hierarchically porous Au-Cu films
Materials Letters 71 (2012) 108–110
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication and electrochemical property of hierarchically porous Au-Cu films Xin-Feng Xing a, Dong-Qing Han a, Yan-Feng Wu a, Yu Guan a, Nan Bao b, Xiao-Hong Xu a,⁎ a b
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 3 November 2011 Accepted 12 December 2011 Available online 19 December 2011 Keywords: Porous materials Metals and alloys Thin films Electrochemical properties
a b s t r a c t The paper presented an alloying/dealloying process for the fabrication of hierarchically porous alloy films (HPAFs). The fabrication process involved electrodeposition of copper on the substrate, annealing to produce new alloy phases, and selective dissolution of copper from the surface. Scanning electron microscopy (SEM) images suggested that the HPAFs were composed of large-sized ligament-channels structures (several micrometers) coupled with small-sized ligament-pores (tens nanometers) interpenetrating the big architectures. Energy dispersive X-ray analysis spectra (EDS) revealed the nonuniform distribution in chemical composition of the large-sized ligament-channel structures. Moreover, we demonstrated that the resulting materials showed catalytic activity for methanol electro-oxidation in alkali solution. We believe that the resulting HPAFs electrode is prospective in the fields of catalysis, sensors, and so on. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Porous metallic materials have recently attracted considerable attention in plenty of applications, including catalysts [1], sensors [2], actuators [3] and fuel cells [4]. During the last decades, dealloying has been proven to be productive and versatile to create sponge-like metals. Considerable efforts were dedicated to the preparation of metallic materials with bicontinuous open structures at nanoscale through dealloying [5]. Most of the previously reported metals with simple microstructures and unimodal pore size distributions were constructed by selective leaching of binary alloys with complete single phase solid solubility across all compositions [6]. Obviously, materials with multimodal pore size distributions are of greater interest for varied technological applications. In hierarchical structures, larger pores favor mass transfer by reducing transport limitation while smaller ones increase surface area. [7]. Dealloying has been spontaneously applied to create hierarchically porous metals with high-purity in recent years [8]. However, the spongy products should be considered as bimetals or alloys rather than pure metals [9–11]. Furthermore, bimetallic catalysts can enhance reactivity, selectivity and stability remarkably by controlling and modifying their structures and compositions [12–14]. However, despite significant progress has been made in the development of hierarchically porous monometals or unimodal pore-size distribution bimetals, little work has been focused on the construction of foamy alloys structures with a multilevel porosity [3,15].
⁎ Corresponding author. Tel.: + 86 531 88361067; fax: + 86 531 88366512. E-mail address: [email protected] (X.-H. Xu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.12.056
This paper reports on the fabrication of the hierarchically porous alloy films (HPAFs) via alloying/dealloying method. We also investigate the electrochemical performance of the HPAFs electrodes on the direct electrocatalytic oxidation of methanol. Further investigations on the formation mechanism and applications of the HPAFs will be summarized in our next studies. 2. Experimental 2.1. Reagents and Instrumentation The anticorrosive Au80Cu20 (wt.%) alloy wire (0.1 mm in diameter) was purchased from Sino-Platinum Co., Ltd. The morphology of the sample was observed in a field-emission SEM (HITACHI S-5500), and the composition was determined by energy dispersive X-ray spectroscopy (EDX) using a detector (EMAX from Horiba) attached to the microscopes. 2.2. Fabrication of HPAFs Electrodeposition was carried out in an aqueous electrolyte consisting of 0.2 M CuSO4 and 0.5 M H2SO4. Using a classical three-electrode setup with a saturated calomel reference electrode and a Pt counter electrode, copper was potentiostatically deposited (E= −0.2 V) on the wire over 400 s. The geometric area of alloy substrate exposed to the electrolyte was 3.14 mm2. Then the copper-coated wire was annealed in a tubular furnace at 500 °C for 6 h under N2 atmosphere. Consequently, the wire was dealloyed in concentrated HNO3 for 9 h at 30 °C. Finally, residual acid within the pores and channels was thoroughly removed by ultrapure water rinsing.
X.-F. Xing et al. / Materials Letters 71 (2012) 108–110
109
Fig. 1. Low magnification SEM images showing the plan-view (a) and section-view (b) of the dealloyed wire.
2.3. Electrochemical measurements Using the HPAFs electrode as working electrode, cyclic voltammograms (CV) of CH3OH electrooxidation were recorded in mixed solutions of KOH and CH3OH. The solutions were purged with high purity nitrogen for 20 min prior to the tests. 3. Results and discussion Fig. 1 shows the plan-view and section-view SEM images of the samples after a series of treatments including electrodeposition, annealing and dealloying. As it is seen from Fig. 1a, the three-step technique generated a highly porous structure throughout the surface of the wire. In order to examine the corrosion depth occurred on the substrate, the sample was cut into two segments so that the crosssectional SEM image could be observed. As shown in Fig. 1b, the edge of the section was found to be rough, and the thickness of the porous portion was estimated to be approximately 2 μm. Additionally, the smooth and pale area marked with an ellipse (Fig. 1a) was uncovered substrate and this could be considered as a verification of the anticorrosive property of the starting material. The results above demonstrate that the dealloying process just took place on the surface and the consequent porous film was thin. As shown in Fig. 2a and b, the high magnification SEM images display a morphology composed of hierarchically porous structures. The large-
sized ligament-channel morphology exhibits an open and bicontinuous structure with large pores appearing in the channels. The sizes of the porous ligaments range from 0.5 to 1 μm and the micropores have a diameter of about 1 μm. Fig. 2b shows a typical large-sized pore, which suggests that the spongy ligament-channel structures are composed of nanoscale ligaments and pores. The small-sized ligament-pore morphology presents a NPG-like structure, which has a three-dimensional interpenetrating framework with clean and smooth surfaces. The nano-ligaments show an average size around 70 nm, while the nanopores exhibit a diameter of 50 nm. Fig. 2c and d exhibit the EDX spectra corresponding to the compositional information over different areas of the large-sized poreligament structures. The EDX spectra indicate that partial copper atoms have been selectively leached out from the surface of the asannealed alloy. The site “A” as marked in Fig. 2b is a typical location showing the pore area, and the EDX analysis suggests that the average mass ratio of Au:Cu is about 52:48 in this region. The site “B” is a typical location displaying the porous ligament, and the average mass ratio is approximately 67:33. It is significant to note that the element content within the large pore differs from the porous ligament area. Fig. 3 presents the voltammetric behaviors of the as-prepared HPAFs and the original alloy in 0.5 M H2SO4 solution at a scan rate of 20 mV s − 1. The first cyclic potential scan (curve a) of the HPAFs electrode contains distinct peaks at voltages between 0.26 and
Fig. 2. High magnification SEM images (a) and (b) showing hierarchically porous structures; EDX spectra for the pore region (c) and ligament region (d) of a large pore.
110
X.-F. Xing et al. / Materials Letters 71 (2012) 108–110
As mentioned above, the oxidation process began with the generation of gold pre-oxidation species during the anodic scan. Then the oxidation of methanol and the reduction of partial gold oxides happened at the same time at a higher potential. Additionally, the inset of Fig. 4 exhibits a linear and increasing relationship between the oxidation peak current density and methanol concentration. The related results have been published in the previous studies [17]. Good transport channels and abundant active sites provided by the HPAFs may greatly facilitate the catalytic oxidation of methanol. However, several problems remain to be investigated, for example, the role copper element played in the electrooxidation process. 4. Conclusion Fig. 3. CV curves recorded in N2-saturated 0.5 M H2SO4 for (a) the original wire, (b) the first cycle and (c) the stable state of HPAFs.
To sum up, the alloying/dealloying method is an efficient way to fabricate hierarchically porous alloy films (HPAFs) with high activity to the electrooxidation of methanol. The as-prepared HPAFs exhibit welldefined structures with large-sized ligament-channels structures (several micrometers) and small-sized ligament-pores (tens nanometers) interpenetrating the big architectures. This work also provides a simple pathway for the design and fabrication of novel hierarchically porous materials. Supplementary materials related to this article can be found online at doi:10.1016/j.matlet.2011.12.056. Acknowledgments This work was supported by the National Science Foundation of China (20776079, 21176144), the Science and Technology Development Project of Shandong Province (2010GGX10314), and the Natural Science Foundation of Shandong Province (ZR2011BM005).
Fig. 4. CV curves of 0, 0.5, 1.0, 1.5 and 2 M methanol in 0.5 M of KOH on HPAFs, scan rate = 20 mV s− 1. The inset was the calibration plot of CV oxidation peak current density versus concentration of methanol.
0.54 V, corresponding to the oxidation and reduction of copper. This demonstrates that a small quantity of copper atoms remain on the surface of the HPAFs composite. Curve b provides the information describing the stable state of the HPAFs electrode after several cycles. The curve suggests that the copper oxidation and reduction peaks gradually reduced to a little constant and the electrode behaved similarly to a typical gold-based electrode. Fig. 4 shows the CV curves of HPAFs with different methanol concentrations in a 0.5 M KOH solution. In KOH solution without methanol, an oxidation peak at 0.23 V can be attributed to the formation of surface gold oxides. According to the previous studies [16], OH − anions strongly adsorbed on the electrode and oxided methanol molecules through a mediated electron transfer mechanism. In the alkaline solutions containing methanol, the onset potential of methanol oxidation occurred at −0.08 V while the large current peaks around 0.2 V represented the catalytic oxidation of methanol. The oxidation current density on the HPAFs electrode is much higher than that of the original alloy electrode (Fig. S1, Supporting Information).
References [1] Zielasek V, Jürgens B, Schulz C, Biener J, Biener MM, Hamza AV, et al. Angew Chem Int Ed 2006;45:8241–4. [2] Wittstock A, Biener J, Bäumer M. Phys Chem Chem Phys 2010;12:12919–30. [3] Jin HJ, Wang XL, Parida S, Wang K, Seo M, Weissmüller J. Nano Lett 2010;10: 187–94. [4] Alia SM, Zhang G, Kisailus D, Li D, Gu S, Jensen K, et al. Adv Funct Mater 2010;20: 3742–6. [5] Erlebacher J, Aziz MJ, Karma A, Dimitrov N, Sieradzki K. Nature 2001;410:450–3. [6] Ding Y, Chen MW. MRS Bull 2009;34:569–76. [7] Ding Y, Erlebacher J. J Am Chem Soc 2003;125:7772–3. [8] Liu WB, Zhang SC, Li N, Zheng JW, Xing YL. Microporous Mesoporous Mater 2011;138:1–7. [9] Haruta M. ChemPhysChem 2007;8:911–3. [10] Wittstock A, Neumann B, Schaefer A, Dumbuya K, Kübel C, Biener MM, et al. J Phys Chem C 2009;113:5593–600. [11] Moskaleva LV, Röhe S, Wittstock A, Zielasek V, Klüner T, Neymancd KM, et al. Phys Chem Chem Phys 2011;13:4529–39. [12] Scott RWJ, Datye AK, Crooks RM. J Am Chem Soc 2003;125:3708–9. [13] Ferrando R, Jellinek J, Johnson RL. Chem Rev 2008;108:845–910. [14] Peng XH, Pan QM, Rempelb GL. Chem Soc Rev 2008;37:1619–28. [15] Lang XY, Guo H, Chen LY, Kudo A, Yu JS, Zhang W, et al. J Phys Chem C 2010;114: 2600–3. [16] Assiongbon KA, Roy D. Surf Sci 2005;594:99–119. [17] Chen L, Fujita T, Ding Y, Chen MW. Adv Funct Mater 2010;20:2279–85.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication and electrochemical property of hierarchically porous Au-Cu films Xin-Feng Xing a, Dong-Qing Han a, Yan-Feng Wu a, Yu Guan a, Nan Bao b, Xiao-Hong Xu a,⁎ a b
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 3 November 2011 Accepted 12 December 2011 Available online 19 December 2011 Keywords: Porous materials Metals and alloys Thin films Electrochemical properties
a b s t r a c t The paper presented an alloying/dealloying process for the fabrication of hierarchically porous alloy films (HPAFs). The fabrication process involved electrodeposition of copper on the substrate, annealing to produce new alloy phases, and selective dissolution of copper from the surface. Scanning electron microscopy (SEM) images suggested that the HPAFs were composed of large-sized ligament-channels structures (several micrometers) coupled with small-sized ligament-pores (tens nanometers) interpenetrating the big architectures. Energy dispersive X-ray analysis spectra (EDS) revealed the nonuniform distribution in chemical composition of the large-sized ligament-channel structures. Moreover, we demonstrated that the resulting materials showed catalytic activity for methanol electro-oxidation in alkali solution. We believe that the resulting HPAFs electrode is prospective in the fields of catalysis, sensors, and so on. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Porous metallic materials have recently attracted considerable attention in plenty of applications, including catalysts [1], sensors [2], actuators [3] and fuel cells [4]. During the last decades, dealloying has been proven to be productive and versatile to create sponge-like metals. Considerable efforts were dedicated to the preparation of metallic materials with bicontinuous open structures at nanoscale through dealloying [5]. Most of the previously reported metals with simple microstructures and unimodal pore size distributions were constructed by selective leaching of binary alloys with complete single phase solid solubility across all compositions [6]. Obviously, materials with multimodal pore size distributions are of greater interest for varied technological applications. In hierarchical structures, larger pores favor mass transfer by reducing transport limitation while smaller ones increase surface area. [7]. Dealloying has been spontaneously applied to create hierarchically porous metals with high-purity in recent years [8]. However, the spongy products should be considered as bimetals or alloys rather than pure metals [9–11]. Furthermore, bimetallic catalysts can enhance reactivity, selectivity and stability remarkably by controlling and modifying their structures and compositions [12–14]. However, despite significant progress has been made in the development of hierarchically porous monometals or unimodal pore-size distribution bimetals, little work has been focused on the construction of foamy alloys structures with a multilevel porosity [3,15].
⁎ Corresponding author. Tel.: + 86 531 88361067; fax: + 86 531 88366512. E-mail address: [email protected] (X.-H. Xu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.12.056
This paper reports on the fabrication of the hierarchically porous alloy films (HPAFs) via alloying/dealloying method. We also investigate the electrochemical performance of the HPAFs electrodes on the direct electrocatalytic oxidation of methanol. Further investigations on the formation mechanism and applications of the HPAFs will be summarized in our next studies. 2. Experimental 2.1. Reagents and Instrumentation The anticorrosive Au80Cu20 (wt.%) alloy wire (0.1 mm in diameter) was purchased from Sino-Platinum Co., Ltd. The morphology of the sample was observed in a field-emission SEM (HITACHI S-5500), and the composition was determined by energy dispersive X-ray spectroscopy (EDX) using a detector (EMAX from Horiba) attached to the microscopes. 2.2. Fabrication of HPAFs Electrodeposition was carried out in an aqueous electrolyte consisting of 0.2 M CuSO4 and 0.5 M H2SO4. Using a classical three-electrode setup with a saturated calomel reference electrode and a Pt counter electrode, copper was potentiostatically deposited (E= −0.2 V) on the wire over 400 s. The geometric area of alloy substrate exposed to the electrolyte was 3.14 mm2. Then the copper-coated wire was annealed in a tubular furnace at 500 °C for 6 h under N2 atmosphere. Consequently, the wire was dealloyed in concentrated HNO3 for 9 h at 30 °C. Finally, residual acid within the pores and channels was thoroughly removed by ultrapure water rinsing.
X.-F. Xing et al. / Materials Letters 71 (2012) 108–110
109
Fig. 1. Low magnification SEM images showing the plan-view (a) and section-view (b) of the dealloyed wire.
2.3. Electrochemical measurements Using the HPAFs electrode as working electrode, cyclic voltammograms (CV) of CH3OH electrooxidation were recorded in mixed solutions of KOH and CH3OH. The solutions were purged with high purity nitrogen for 20 min prior to the tests. 3. Results and discussion Fig. 1 shows the plan-view and section-view SEM images of the samples after a series of treatments including electrodeposition, annealing and dealloying. As it is seen from Fig. 1a, the three-step technique generated a highly porous structure throughout the surface of the wire. In order to examine the corrosion depth occurred on the substrate, the sample was cut into two segments so that the crosssectional SEM image could be observed. As shown in Fig. 1b, the edge of the section was found to be rough, and the thickness of the porous portion was estimated to be approximately 2 μm. Additionally, the smooth and pale area marked with an ellipse (Fig. 1a) was uncovered substrate and this could be considered as a verification of the anticorrosive property of the starting material. The results above demonstrate that the dealloying process just took place on the surface and the consequent porous film was thin. As shown in Fig. 2a and b, the high magnification SEM images display a morphology composed of hierarchically porous structures. The large-
sized ligament-channel morphology exhibits an open and bicontinuous structure with large pores appearing in the channels. The sizes of the porous ligaments range from 0.5 to 1 μm and the micropores have a diameter of about 1 μm. Fig. 2b shows a typical large-sized pore, which suggests that the spongy ligament-channel structures are composed of nanoscale ligaments and pores. The small-sized ligament-pore morphology presents a NPG-like structure, which has a three-dimensional interpenetrating framework with clean and smooth surfaces. The nano-ligaments show an average size around 70 nm, while the nanopores exhibit a diameter of 50 nm. Fig. 2c and d exhibit the EDX spectra corresponding to the compositional information over different areas of the large-sized poreligament structures. The EDX spectra indicate that partial copper atoms have been selectively leached out from the surface of the asannealed alloy. The site “A” as marked in Fig. 2b is a typical location showing the pore area, and the EDX analysis suggests that the average mass ratio of Au:Cu is about 52:48 in this region. The site “B” is a typical location displaying the porous ligament, and the average mass ratio is approximately 67:33. It is significant to note that the element content within the large pore differs from the porous ligament area. Fig. 3 presents the voltammetric behaviors of the as-prepared HPAFs and the original alloy in 0.5 M H2SO4 solution at a scan rate of 20 mV s − 1. The first cyclic potential scan (curve a) of the HPAFs electrode contains distinct peaks at voltages between 0.26 and
Fig. 2. High magnification SEM images (a) and (b) showing hierarchically porous structures; EDX spectra for the pore region (c) and ligament region (d) of a large pore.
110
X.-F. Xing et al. / Materials Letters 71 (2012) 108–110
As mentioned above, the oxidation process began with the generation of gold pre-oxidation species during the anodic scan. Then the oxidation of methanol and the reduction of partial gold oxides happened at the same time at a higher potential. Additionally, the inset of Fig. 4 exhibits a linear and increasing relationship between the oxidation peak current density and methanol concentration. The related results have been published in the previous studies [17]. Good transport channels and abundant active sites provided by the HPAFs may greatly facilitate the catalytic oxidation of methanol. However, several problems remain to be investigated, for example, the role copper element played in the electrooxidation process. 4. Conclusion Fig. 3. CV curves recorded in N2-saturated 0.5 M H2SO4 for (a) the original wire, (b) the first cycle and (c) the stable state of HPAFs.
To sum up, the alloying/dealloying method is an efficient way to fabricate hierarchically porous alloy films (HPAFs) with high activity to the electrooxidation of methanol. The as-prepared HPAFs exhibit welldefined structures with large-sized ligament-channels structures (several micrometers) and small-sized ligament-pores (tens nanometers) interpenetrating the big architectures. This work also provides a simple pathway for the design and fabrication of novel hierarchically porous materials. Supplementary materials related to this article can be found online at doi:10.1016/j.matlet.2011.12.056. Acknowledgments This work was supported by the National Science Foundation of China (20776079, 21176144), the Science and Technology Development Project of Shandong Province (2010GGX10314), and the Natural Science Foundation of Shandong Province (ZR2011BM005).
Fig. 4. CV curves of 0, 0.5, 1.0, 1.5 and 2 M methanol in 0.5 M of KOH on HPAFs, scan rate = 20 mV s− 1. The inset was the calibration plot of CV oxidation peak current density versus concentration of methanol.
0.54 V, corresponding to the oxidation and reduction of copper. This demonstrates that a small quantity of copper atoms remain on the surface of the HPAFs composite. Curve b provides the information describing the stable state of the HPAFs electrode after several cycles. The curve suggests that the copper oxidation and reduction peaks gradually reduced to a little constant and the electrode behaved similarly to a typical gold-based electrode. Fig. 4 shows the CV curves of HPAFs with different methanol concentrations in a 0.5 M KOH solution. In KOH solution without methanol, an oxidation peak at 0.23 V can be attributed to the formation of surface gold oxides. According to the previous studies [16], OH − anions strongly adsorbed on the electrode and oxided methanol molecules through a mediated electron transfer mechanism. In the alkaline solutions containing methanol, the onset potential of methanol oxidation occurred at −0.08 V while the large current peaks around 0.2 V represented the catalytic oxidation of methanol. The oxidation current density on the HPAFs electrode is much higher than that of the original alloy electrode (Fig. S1, Supporting Information).
References [1] Zielasek V, Jürgens B, Schulz C, Biener J, Biener MM, Hamza AV, et al. Angew Chem Int Ed 2006;45:8241–4. [2] Wittstock A, Biener J, Bäumer M. Phys Chem Chem Phys 2010;12:12919–30. [3] Jin HJ, Wang XL, Parida S, Wang K, Seo M, Weissmüller J. Nano Lett 2010;10: 187–94. [4] Alia SM, Zhang G, Kisailus D, Li D, Gu S, Jensen K, et al. Adv Funct Mater 2010;20: 3742–6. [5] Erlebacher J, Aziz MJ, Karma A, Dimitrov N, Sieradzki K. Nature 2001;410:450–3. [6] Ding Y, Chen MW. MRS Bull 2009;34:569–76. [7] Ding Y, Erlebacher J. J Am Chem Soc 2003;125:7772–3. [8] Liu WB, Zhang SC, Li N, Zheng JW, Xing YL. Microporous Mesoporous Mater 2011;138:1–7. [9] Haruta M. ChemPhysChem 2007;8:911–3. [10] Wittstock A, Neumann B, Schaefer A, Dumbuya K, Kübel C, Biener MM, et al. J Phys Chem C 2009;113:5593–600. [11] Moskaleva LV, Röhe S, Wittstock A, Zielasek V, Klüner T, Neymancd KM, et al. Phys Chem Chem Phys 2011;13:4529–39. [12] Scott RWJ, Datye AK, Crooks RM. J Am Chem Soc 2003;125:3708–9. [13] Ferrando R, Jellinek J, Johnson RL. Chem Rev 2008;108:845–910. [14] Peng XH, Pan QM, Rempelb GL. Chem Soc Rev 2008;37:1619–28. [15] Lang XY, Guo H, Chen LY, Kudo A, Yu JS, Zhang W, et al. J Phys Chem C 2010;114: 2600–3. [16] Assiongbon KA, Roy D. Surf Sci 2005;594:99–119. [17] Chen L, Fujita T, Ding Y, Chen MW. Adv Funct Mater 2010;20:2279–85.