ARTICLE pubs.acs.org/JPCC
High-Density Nanoporous Structures for Enhanced Electrocatalysis Jaeyoung Lee,*,†,‡ HyungKuk Ju,† Youngmi Yi,‡ Jongmin Lee,† Sunghyun Uhm,† Jae Kwang Lee,† and Hye Jin Lee*,§ †
Ertl Center for Electrochemistry and Catalysis/RISE & School of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany § Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, South Korea
bS Supporting Information ABSTRACT: A novel method to create a high-density nanopore structure between the neighboring pores of anodic alumina template is developed via applying a simple consecutive three-step anodization process. As a demonstration, the nanostructure comprising more numbers of individually addressable Pd nanorods was utilized to enhance the electrocatalysis of ethanol in alkaline media, which is due to a dramatic increment in surface reactive sites.
’ INTRODUCTION Of late, one of the most well-controllable methods to fabricate nanostructures and nano-objects for bioengineering and energy applications is template-based studies utilizing either polymer membranes1,2 or ceramic membranes.38 The former has received great attention due to their convenient use, but the poor thermal stability has limited their versatile applicabilities, whereas the latter, for instance, self-ordered nanopores in anodic aluminum oxide (AAO) membrane, offer higher thermal and chemical stabilities without any defects in addition to tunable nanopore dimensions such as variable diameters with high aspect ratio. Since a 10% porosity rule of AAO template has been reported by Nielsch et al.,4 there have been only a few reports on enhancing the nanopore density of AAO membrane.5,6 Besides improved physicochemical and electronic properties of nanosized objects altering its activity and durability7,8 in (electro)catalytic reactions, more highly densed active surface sites in limited geometric areas and volumes of the nanomaterials are required to improve utilization efficiency of electrocatalyst keeping accessibility of reactants in the field of electrocatalytic oxidation of small organic molecules, water electrolysis, and fuel cells.914 Herein we demonstrate a novel method to create highly densed nanopores via reducing regularly an interdistance between the neighboring pores of AAO membranes. This was achieved by applying a three-step consecutive anodization processes involving the preparation of (i) the largest nanopore in H3PO4, (ii) the smallest nanopore in H2SO4, and (iii) uniform growing of smallest nanopore. In particular, an array of densely filled nanopores with Pd was developed for the more efficient electrocatalytic oxidation of ethanol that can further be applied to liquid fuel cell devices.
for 4 min in a mixed solution of HClO4 and C2H5OH with a volume ratio of 1:3. Then, the electropolished Al sheet was anodized under a constant voltage of 195 V for 24 h in 1 wt % H3PO4 at a temperature of 2 °C. The alumina layers that formed during first anodization were selectively removed in a mixture of 6 wt % H3PO4 and 1.8 wt % H2CrO4 for 24 h in water bath with a temperature of 50 °C. Subsequently, textured aluminum sheet with concave shape was anodized under a constant voltage of 25 V for 1 h in 0.3 M H2SO4 at a temperature of 8 °C; then, the formed nanoporous alumina film was selectively removed. Al sheet anodized from different electrolytic acids was finally anodized under a constant voltage of 25 V for 10 h in 0.3 M H2SO4 at a temperature of 2 °C. (See Table 1 of the Supporting Information for detailed conditions.) Moreover, to confirm the ability of metallic nanostructures formed by this alumina template, Pd nanorod arrays were fabricated into AAO template. After the fabrication of both nanochannel structured SS AAO and PSS AAO, Au was coated onto each nanostructure by applying direct current (DC) sputtering to conduct electricity. A constant potential of 0.8 V (vs Ag/AgCl), that is, a total charge of 250 mC, was applied for the growth of Pd nanorod in 10 mM Pd(NH3)4Cl2 and 5 mM Na2EDTA (ethylenediaminetetraacetic acid). To reveal Pd nanorod array on Au layer, we dissolved aluminum oxide in 2 M NaOH for 5 h. Finally, the ethanol electrooxidation of Pd nanorod arrays obtained from SS AAO and PSS AAO were carried out using cyclic voltammetry in 1.0 M KOH and 1.0 M C2H5OH. The electrodeposition and the electrochemical analysis were performed in a conventional three-electrode electrochemical cell. Pt wire as counter electrode, Ag/AgCl/3 M KCl
’ EXPERIMENTAL SECTION Prior to first anodization, an aluminum sheet (99.999%, GoodFellow) was electropolished under a constant voltage of 20 V
Received: September 23, 2011 Revised: December 23, 2011 Published: December 27, 2011
r 2011 American Chemical Society
2915
dx.doi.org/10.1021/jp2091889 | J. Phys. Chem. C 2012, 116, 2915–2918
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Comparison of pore density and pore size distribution between SS AAO and PSS AAO. Number of pores and size distribution were determined in a unit area of 4 μm2.
Figure 1. (a) SEM image of SS AAO prepared by two-step anodization in 0.3 M H2SO4, (b) top surface view of PS AAO sequentially anodized from 1 wt % H3PO4 as the first anodization and 0.3 M H2SO4 as the second anodization (inset: cross-sectional view of PS AAO), and (c) top surface view of PSS AAO consecutively anodized from 1 wt % H3PO4 as the first anodization and 0.3 M H2SO4 as the second and third anodization (inset: cross-sectional view).
for Pd electrodeposition, and Hg/HgO for ethanol electrooxidation as the reference electrode were employed. A potentiostat/galvanostat (Autolab, PGSTAT1287N) was used for electrochemical measurements.
’ RESULTS AND DISCUSSION An SEM image of AAO prepared by normal two-step anodization in 0.3 M H2SO4 (hereafter SS AAO) is shown in Figure 1a. The ordered porous structures featuring the interspace between the pores of 60 nm and uniform pore diameter of 1820 nm resulted in the pore density of ∼3.0 1010 /cm2. To break 10% porosity rule,4 we applied new consecutive anodization steps in 1 wt % H3PO4 at the first anodization and 0.3 M H2SO4 at the second anodization (hereafter PS AAO). Figure 1b shows the selected magnified image of PS AAO, where many tiny and less ordered nanopores in hexagonally textured larger boundary were observed. We understood that the hexagonally textured area was first created during the first anodization, and subsequent second anodization makes porous hole structure inside hexagonally shaped texture area. The pore density of PS AAO is slightly higher than the one of SS AAO. However, the inset of Figure 1b exhibits a complicated cross-sectional view of concave-shaped textures PS AAO. As mentioned, these textures were formed during the first anodization in H3PO4 after selective dissolution of alumina matrix, and some of pores were perpendicularly wellgrown inside the texture during second anodization in H2SO4. These phenomena mainly occurred in the neighborhood of boundaries at concaved-shaped textured areas due to the repulsive force of pores. We assume that the uniform grown pores with higher density might be possible by further anodization in 0.3 M H2SO4
(hereafter PSS AAO). Figure 1c and the inset image clearly show more densed perpendicular pores of PSS AAO, and compared with PS AAO, better ordered structures and wider pores were also observed. A porosity analysis of PSS and SS AAO membrane was performed to clarify the difference between PSS AAO and PS AAO (Figure 2). In a pore size distribution, general SS AAO shows slightly narrower regime indicating relatively uniform pore size distribution with similar diameter of 1820 nm. The pore size of PSS AAO possessed relatively broader range of a diameter between 20 and 25 nm. The possible reactive surface could be improved because the pore density of PSS AAO was ∼4.5 1010/cm2, which is ∼50% higher than that of SS AAO. The increase in pore numbers can be explained by the fact that relatively tiny pores were formed in the boundary of hexagonally concaved textures, leading to the slightly reduced interpore distance (
