Galvanic synthesis of three-dimensional and hollow metallic nanostructures
© Park et al.; licensee Springer. 2014
Received: 19 September 2014
Accepted: 4 December 2014
Published: 16 December 2014
We report a low-cost, facile, and template-free electrochemical method of synthesizing three-dimensional (3D) hollow metallic nanostructures. The 3D nanoporous gold (3D-NPG) nanostructures were synthesized by a galvanic replacement reaction (GRR) using the different reduction potentials of silver and gold; hemispherical silver nanoislands were electrochemically deposited on cathodic substrates by a reverse-pulse potentiodynamic method without templates and then nanoporous gold layer replicated the shape of silver islands during the GRR process in an ultra-dilute electrolyte of gold(III) chloride trihydrate. Finally, the wet etching process of remaining silver resulted in the formation of 3D-NPG. During the GRR process, the application of bias voltage to the cathode decreased the porosity of 3D-NPG in the voltage range of 0.2 to -0.62 V. And the GRR process of silver nanoislands was also applicable to fabrication of the 3D hollow nanostructures of platinum and palladium. The 3D-NPG nanostructures were found to effectively enhance the SERS sensitivity of rhodamine 6G (R6G) molecules with a concentration up to 10-8 M.
Nanoporous gold (NPG) structures have received a great deal of attention due to their potential applications in the fields of double layer capacitors, fuel cells, biosensors, electrocatalysis, etc. [1–4]. Many researchers have studied nanoporous metallic materials with a large specific surface area (i.e., narrow pore-size distributions), because the nanoporosity affects not only active sites and electron mobility in the solid ligaments but also geometrical confinement effects [4–8]. It was reported that Pt-coated NPG nanostructures with smaller pore-size exhibited a higher catalytic activity in methanol oxidation than those with larger pore-size [9–11]. Recently, it was demonstrated that the surface enhanced Raman spectroscopy (SERS) effects of NPG depended on the pore size, the ratios of ligaments to nanopores, and the surface roughness [12, 13].
The typical methods of synthesizing NPG structures have been electrodeposition, anodization, and dealloying processes [14–16]. Generally, three-dimensional (3D) structures of NPG have been fabricated using the hard or soft templates of porous membranes and self-assembled spheres of dielectric/conductive materials [9–11, 17–20]. Recently, dual-templates of porous alumina and polystyrene microspheres were utilized to fabricate hierarchical macro/mesoporous gold wires . Due to the combination of high specific surface area and easy transport of reactants in an electrochemical system, the 3D structures of modified NPG have attracted growing interest for applications such as electrocatalysis, energy conversion, and energy storage [5, 22]. Among these structures, 3D hollow structures of NPG have attracted more attention due to their high surface area, low density, usable nanoscale inner space, and unusual characteristics determined by shape and composition [23, 24]. However, the template-based process of fabricating 3D nanostructures is still complicated and time-consuming. More recently, for the low-cost mass production of NPG structures, Jiao et al. reported the patterned NPG two-dimensional array was produced by a straightforward imprinting process .
Here, we have developed a facile and low-cost electrochemical method of fabricating 3D hollow nanostructures of nanoporous gold, based on the filamentary deposition and galvanic reduction reaction without templates or surfactants . The present method was able to be applied to fabricate 3D hollow nanostructures of gold, platinum, and palladium. And it is demonstrated that the 3D nanoporous gold (3D-NPG) nanostructures, as uniform SERS substrate without hot spots, exhibit a higher SERS enhancement factor than planar nanoporous gold films.
Fabrication of 3D hollow nanostructures
The silver nanoislands were electrodeposited in an electrolyte of 20 μM AgNO3 (#209139, reagent A.C.S., Sigma-Aldrich, St. Louis, MO, USA) and 2.11 mM NH4OH (#13370-0380, guaranteed reagent, Junsei, Tokyo, Japan) using a reverse-pulse potentiodynamic electrochemical method . The reverse-pulse potentiodynamic process was performed at a reduction potential (VR) of 14 V and oxidation potential (VO) of 0.5 V for 2 h at the frequency of 0.5 Hz using the electrochemical system (Solartron, Model 1280z). Three electrodes were used; a Pt wire (0.5-mm in diameter and 1-m in length, Sigma-Aldrich) as a counter electrode, a KCl-saturated Ag/AgCl electrode as a reference electrode, and a sputtered Au film (90 nm in thickness) on a Si wafer as a working electrode. Galvanic replacement reaction (GRR) of the silver nanoislands was performed in an elect rolyte of 50 μM HAuCl4 · nH2O (n = 3.5, Kojima Chemicals Co., #903060) for 10, 24, 48, and 72 h without stirring. The GRR process resulted in the formation of core/shell structures, i.e., silver core and gold shell, because the silver atoms on the hemispherical silver islands were replaced by gold atoms. Then, a selective etching process of the silver core was performed in a 7.5 M HNO3 solution. And a process was carried out in a NH4OH solution (28 to 30 vol. %) to remove AgCl precipitates formed during the GRR process. The bias voltages of 0.2, -0.3, and -0.62 V was applied between the cathodic specimen and anodic Pt wire during the GRR process for 24 h in order to control the porosity of 3D-NPG nanostructures.
Three-dimensional platinum nanostructures with a nanoporosity were synthesized by the subsequent GRR process of the same silver nanoislands in the electrolyte of 50 μM H2PtCl6 · H2O (#254029, 99.995% trace metals basis, Sigma-Aldrich) while three-dimensional palladium nanostructures were produced by the GRR process in the electrolyte of 100 μM Na2PdCl4 (#379808, 99.99% trace metals basis, Sigma-Aldrich) at the bias voltage of -0.6 V.
Planar nanoporous gold (PNPG) films were synthesized on silver films (60 nm in thickness) which were sputter-deposited on Au coated Si substrates. The GRR process was performed in an electrolyte of 50 μM HAuCl4 · nH2O for 12 h without a bias voltage. And then the selective etching process of sliver atoms led to the formation of the PNPG films.
The morphologies and crystal structures of the 3D nanostructures were analyzed by a field-emission scanning electron microscope (SEM; Hitachi S-4800, Hitachi Ltd, Chiyoda-ku, Japan) and a high resolution transmission electron microscope (HRTEM; FEI Tecnai G2 F30, 300 kV, FEI, Minato-ku, Japan) equipped with an energy dispersive X-ray spectroscopy detector (EDS; EDAX Inc., Kanagawa, Japan), respectively. The relative electrochemical surface areas (rESA) of the nanostructures were evaluated by measuring the charge quantity consumed for the reduction of the surface oxide layer using a cyclic voltammogram at a scan rate of 50 mV∙s-1 in a N2-saturated electrolyte of 0.1 M H2SO4. The SERS measurements were performed using a homemade micro-Raman system based on a 633 nm He/Ne laser and a 100 × objective lens with a thermoelectrically cooled CCD detector (iDUS 401, ANDOR, Belfast, Northern Ireland, UK). The laser power was approximately 0.25 mW and the focused spot size was 1 μm. The spectra were obtained at random locations for each sample with an integration time of 10 s for all the measurements. In order to conduct SERS measurements, the nanostructures on substrates were immersed in a rhodamine 6G (R6G) aqueous solution with a concentration in the range of 10-4 to 10-8 M, rinsed thoroughly with high purity water for the formation of mono-dispersed R6G molecules, and then dried under blowing N2.
Results and discussion
3D nanoporous gold structures
AuCl4- is reduced to one Au atom and silver is oxidized to form AgCl. The GRR generates the small pits on silver nanoislands (marked by the yellow arrows in Additional file 1: Figure S1a), which have relatively high surface energy [20, 27]. During the GRR process, gold ions are expected to be reduced forming facetted surfaces because gold and silver phases have the same face-centered crystal structure, with lattice constants of 4.0786 and 4.0862 Å, respectively [20, 27]. Thus, a thin Au facetted layer is formed replicating the surface of silver nanoislands and prevents underlying silver from reacting with AuCl4- ion.
The etching process occurs through the small pits, which play a role of the diffusion pathway for Ag+ (as indicated by arrows in Additional file 1: Figure S1a-d). During the etching process, silver in the core and silver existing in the thin Au shell layer are selectively dissolved and resulted in the formation of hollow and nanoporous Au shell structure, i.e., 3D nanoporous gold structure. The nanoporosity might be ascribed that AuCl4- is reduced to one gold atom at the expense of three silver atoms, according to Equation 1. As the surface area and surface energy increase with further etching of silver, the reconstruction of the pore morphology occurs via Ostwald ripening process [27, 28].
3D-nanoshell gold structures
3D-nanostructures of platinum and palladium
With a similar vein, 3D-nanoshell palladium nanostructures were fabricated by the GRR process in 100 μM Na2PdCl4 (bias voltage of -0.6 V) and a selective etching process, as shown in Figure 6b. It was noted that the 3D-nanoshell palladium structures had thick shells rather than porous structure with the help of bias voltage, as discussed above.
SERS of 3D nanoporous gold structures
We suggest a facile and low-cost process of fabricating 3D hollow metallic nanostructures. The 3D hollow nanostructures of gold, platinum, and palladium were uniformly synthesized using sacrificial silver nanostructures on a substrate. As the porosity of the 3D hollow nanostructures was able to be controlled by the bias voltage between the cathode and the anode, the nanoporous and nanoshell structures could be alternatively synthesized. In comparison with PNPG nanostructures, the 3D-NPG nanostructures were proved to be a superior SERS substrate with a higher enhancement of SERS intensity. The template-free electrodeposition and GRR process based on ultra-dilute electrolytes are expected to be utilized for further development of various 3D hollow porous nanostructures.
We would like to acknowledge the financial support from the R&D Convergence Programs of MSIP (Ministry of Science, ICT and Future Planning), ISTK (Korea Research Council for Industrial Science and Technology) (Grant B551179-13-01-03), and KRCF (Korea Research Council of Fundamental Science and Technology) (Grant 13-2-KIST) of Republic of Korea.
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