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Controllable Synthesis of Hollow Hierarchical Palladium Nanostructures with Enhanced Activity for Proton/Hydrogen Sensing Han-Pu Liang,*,† Nathan S. Lawrence,† Li-Jun Wan,‡ Li Jiang,§ Wei-Guo Song,‡ and Timothy G. J. Jones*,† Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0EL, UK, Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, People’s Republic of China, and Schlumberger-Doll Research, 1 Hampshire Street, Cambridge, Massachusetts 02139 ReceiVed: July 5, 2007; In Final Form: October 16, 2007
The hollow Pd nanostructures were fabricated using Co nanoparticles as sacrificial templates to obtain two geometries, namely, raspberry-like and nanotube-like hierarchical architectures. These hollow nanostructures are extensively characterized by TEM, XRD, HRTEM, SEM, and energy dispersive X-ray analysis. Particularly, hollow raspberry-like Pd nanostructure exhibit enhanced activity for proton/hydrogen sensing compared to solid Pd nanoparticles (diameter ca. 3 nm), Pd microparticles (diameter 1-1.5 µm), and a planar 3 mm diameter Pd electrode. It is the contribution of the hollow nanostructure and the presence of the smaller nanoparticles on its shell that allow the weakly capped structure to produce this stronger and more specific stable response.
Studies on the interaction between metals and hydrogen have become increasingly important, driven by the need to use hydrogen as an alternative energy source to relieve greenhouse gas emissions and the threat of an energy crisis.1 Palladium is one of most popular metals used in chemical sensors for the detection of hydrogen gas and as a hydrogen storage material, since hydrogen atoms have a high mobility within the Pd lattice and diffuse rapidly through the metal. The high diffusivity of hydrogen has been utilized beneficially in both the fast detection of hydrogen, with Pd mesowire array sensors, and in the separation of hydrogen from gas mixtures.2 Nanostructured materials have become prominent in scientific research with both their fundamental science and technological applications being studied. Hollow nanostructured metals, with their advantages of high surface area, low density and reduced material costs represent a highly interesting class of materials that exhibit intriguing magnetic, photonic, and catalytic properties different from (or superior to) their solid counterparts.3 To date, there has been only limited success in the preparation of hollow Pd nanostructures owing to their intrinsic difficulties in fabrication.4 For example, Pd nanotubes have been successfully fabricated with Ag nanowire,4a,b porous anodic aluminum oxide,4c track-etched polycarbonate,4d and mixed-surfactant liquid crystal4e templates. Hollow Pd microspheres have also been fabricated using silica sphere templates; however, the removal of these templates required the use of 10 M HF.4f Consequently, it is still desirable to develop efficient and costeffective methods to synthesize hollow Pd nanostructures and study their use in a variety of different applications. The predominant advantages of using nanoparticle-modified electrodes compared to typical macroelectrodes are their ex* To whom correspondence should be addressed. Tel: +44 (0)1223 325343. Fax: +44 (0)1223 467004. E-mail: hliang@cambridge. oilfield.slb.com (H.-P.L.),
[email protected] (T.G.J.J.). † Schlumberger Cambridge Research. ‡ Beijing National Laboratory for Molecular Science. § Schlumberger-Doll Research.
tremely small size, unique structures, large effective surface area, increased mass transport, and possible increased electronic interaction between the metal nanoparticles and reactant molecules. This is effectively demonstrated when examining the electrochemical response of hydrogen at various Pd electrodes. In this case, it was found that the electrochemical signal strongly depends on the morphology of the electrode layer.5,6 The use of electrodeposited Pd nanoparticles provided a means of understanding this varying response, as it allowed the oxidation of both the Had and Hab to be deconvolved,7 where Had is an absorbed hydrogen atom, and Hab denotes hydrogen atoms underneath the first atomic layers of Pd atoms from the surface.5 Furthermore, we have recently reported that delicately controlling the structure of Au/Pd core-shell nanoparticles allowed tuning of the electrochemical response such that either the Had or Hab oxidation process could dominate the process.8 This was due to the electrochemical signal being critically dependent on the size and structure of the Pd nanoparticles.8 Herein, we present the controllable synthesis of hollow hierarchical Pd nanostructures by a facile cost-effective method with Co nanoparticles as sacrificial templates. To the best of our knowledge, this is the first report on the controlled hierarchical assembly of hollow Pd nanostructures from small nanoparticles. In particular, we demonstrate that hollow raspberrylike Pd nanostructures exhibit excellent performance in the electrochemical detection of proton/hydrogen. Experimental Section All chemicals were of analytical grade supplied by Aldrich and used as received without further purification. These were palladium(II) chloride (99.999%), Pd microparticles (diameter 1-1.5 µm), sulfuric acid, chloride acid, ethanol, poly(N-vinyl2-pyrrolidone) (PVP, Mw ) 55 000), citric acid, cobalt chloride, and sodium borohydride. All solutions were prepared with deionized water with resistivity of at least 18.2 MΩ cm (Millipore water system).
10.1021/jp0752320 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/15/2007
Synthesis of Hollow Hierarchical Pd Nanostructures The Co nanoparticles were fabricated via an analogous method to that used previously by Kobayashi et al.9 Briefly, under a high-purity N2 atmosphere a solution of CoCl2 (0.4 M, 0.1 mL) was added to a deaerated aqueous solution (100 mL) of NaBH4 (8 mM) and citric acid (0.8 mM). Deareation of the solution was required in order to minimize the possibility of Co nanoparticle oxidation. Freshly synthesized Co nanoparticles produce a black solution, indicating the successful synthesis. Gas was evolved during the reaction, which continued for several minutes. When gas evolution ceased, the resulting solution (100 mL) was immediately added to a stirred solution of H2PdCl4 (1 mM, 40 mL) at room temperature. It should be noted that in all cases the Pd raspberry-like hollow nanoparticles were synthesized with freshly prepared Co nanoparticle solutions in order to minimize oxidation. After addition of the Pd salt, the reaction mixture was centrifuged, to extract the precipitated Pd nanostructures. For the fabrication of tube-like Pd nanostructures the experimental conditions are nearly the same as above, except that the concentration of citric acid was decreased to 0.08 mM. Pd nanoparticles have been successfully prepared by various methods.10 In the present study, solid Pd nanoparticles with an average diameter of 3 nm were prepared by a method described in the literature using the most widely used capping agent.10a Briefly, a 50 mL mixture of aqueous H2PdCl4 (4.0 mM, 7.5 mL, 30 µmol of Pd), ethanol (10 mL), water (32.5 mL), and poly(N-vinyl-2-pyrrolidone) (PVP; 30 µmol) was heated at reflux (100 °C) in a 100-mL flask for 3 h under air to synthesize the PVP-protected Pd nanoparticles. The catalysts loadings were obtained as follows: In the case of hollow raspberry-like Pd nanoparticles, first, it was assumed that all of the PdCl42- was reduced to Pd. A solution containing 2.8 mM Pd solution was obtained by concentrating 2 mL of the original suspension to 0.2 mL by centrifugation, after which a 5 µL aliquot of the sample was placed on the electrode surface and put into an oven at 50 °C to allow solvent evaporation. In the case of solid Pd nanoparticles, a 23.5 µL aliquot of the original suspension was dropped on the electrode surface and allowed to dry. In the case of Pd microparticles, first, a 0.5 mL aqueous suspension containing 3.0 mg of Pd microparticles was prepared and sonicated to aid dispersion. A 0.125 mL aliquot of this suspension was then diluted 4-fold such that the concentration of Pd microparticles was 1.5 mg/mL. Finally, a 10 µL aliquot of this new suspension was placed on the electrode surface and allowed to dry. For TEM and SEM probing, the suspensions were centrifuged and the precipitates were collected, washed, dispersed by ultrasonic treatment, and dropped onto carbon-coated copper grids. TEM measurement and energy dispersive X-ray analysis were performed with a JEM 2010 transmission electron microscopy equipped with an energy dispersive X-ray analyzer (Phoenix). High-resolution TEM images were recorded on a Philips TECNAI F30 operating at 300 kV. Powder X-ray diffractions (XRD) were carried out with a Rigaku D/max-2500 using filtered Cu KR radiation. SEM was carried out with a Hitachi S-4300F field emission scanning electron microscope. The nitrogen sorption isotherm was obtained with an Autosorb-1 system (Quanta Chrome); the sample was degassed for 3 h at 100 °C prior to the measurements being taken. Electrochemical measurements were recorded using a µAutoLab potentiostat (Ecochemie, Netherlands) with a standard three-electrode configuration. Platinum wire (1 mm diameter, Goodfellow Metals, Cambridge, UK) provided the counter electrode and a saturated calomel electrode (SCE, Radiometer,
J. Phys. Chem. C, Vol. 112, No. 2, 2008 339 Copenhagen) acted as the reference. All solutions were degassed with ultrahigh purity nitrogen (BIP N2 Air Products) prior to electrochemical analysis. The working electrode was composed of a boron-doped diamond electrode (3 mm diameter) onto which the Pd particles were immobilized via solvent evaporation. The Pd macroelectrode (3 mm) was purchased from Bioanalytical Systems Ltd (Kenilworth, UK). All references were measured with respect to the saturated calomel electrode. Results and Discussion Figure 1a shows a typical low-magnification TEM image of hollow raspberry-like Pd nanoparticles. There is a strong difference in contrast in nearly all of the spheres, with a bright center surrounded by a much darker edge, confirming their hollow nature. The average size of the nanoparticles estimated from the TEM image is about 80 nm. The high-magnification TEM image in Figure 1b shows an amorphous surface. Upon closer inspection of Figure 1b, it can be seen that there are many smaller nanoparticles on the shell of each individual hollow nanoparticle, forming the raspberry-like structure. The average shell thickness is ca. 15 nm. Figure 1c gives a typical lowmagnification SEM image of the hierarchical Pd architecture, indicating the production of large-scale uniform spherical materials with rough surfaces. A typical high-magnification SEM image of the sample is presented in Figure 1d, in which the raspberry-like structure of the nanoparticles is particularly clear. Powder X-ray diffraction (XRD) analysis was used to characterize the chemical composition and crystal structure of the raspberry-like nanoparticles. A typical XRD pattern of the hollow raspberry-like Pd nanoparticles is shown in Figure 2, where numerous diffraction peaks are recorded. These diffraction peaks are indexed to the diffraction of {111}, {200}, {220}, {311}, and {222} planes of the face-center-cubic (fcc) phase of Pd according to the JCPDS No. 87-0645. Those hollow Pd nanostructures were prepared on the basis of the replacement reaction detailed in eq 1.
Co + PdCl42- ) Pd + Co2+ + 4Cl-
(1)
In this case, the amount of H2PdCl4 present is sufficient to react completely with the added Co nanoparticles on the basis of the stoichiometric relationship as presented in eq 1. The thermodynamic driving force of the reaction is due to the large difference in redox potential between the Co2+/Co and PdCl42-/ Pd couples. Since the standard reduction potential of the PdCl42-/Pd redox couple (0.591 V vs SHE) is greater than that of the Co2+/Co redox couple (-0.277 V vs SHE), PdCl42- is reduced to Pd immediately once the Co nanoparticles are added to the solution. Evidence for this rapid reaction was obtained through the visual change in color of the H2PdCl4 solution as the colloidal Co solution was added; the solution color immediately changed from pale yellow to a black solution. As this replacement reaction occurs rapidly, the reduced Pd atoms nucleate and grow into very small particles, eventually evolving into a thin shell around the cobalt nanoparticles. The shells presumably have an incomplete porous structure at an early stage when both Co2+ and PdCl42- are able to diffuse continuously across the shell in reverse directions.3d Recently, Alivisatos and co-workers used a nanoscale Kirkendall effect to form hollow nanocrystals, where cobalt nanocrystals reacted with oxygen or sulfur to generate cobalt oxide or cobalt sulfide nanoshells.11 As discovered by Kirkendall over half a century ago,12 the difference in the diffusion rates of two components
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Figure 1. Low- and high-magnification TEM (a, b) and SEM (c, d) images of hollow raspberry-like Pd nanoparticles.
Figure 2. A typical powder X-ray diffraction pattern (XRD) of hollow raspberry-like Pd nanoparticles formed by reducing PdCl42- with Co nanoparticles as sacrificial templates.
at the interface results in a net directional flow. In the light of these studies, it is not unreasonable to expect that the hollow Pd nanostructures detailed above are formed via a similar pathway to the Kirkendall effect. The diffusion rate of Co atoms may be relatively fast compared to that of Pd at the newly formed interface between Co and Pd.13 Therefore, as the replacement reaction continues, a net outward movement of Co atoms might be caused. The outward-diffusing Co atoms are immediately oxidized by PdCl42-. The resulting hollow nanostructures formed until the Co nanoparticles were completely
consumed by the replacement reaction. The reason for the formation of raspberry-like structures on the shell is tentatively attributed to the fact that such structures are stable and therefore cannot effectively be reconstructed to form smooth shells as hollow Au nanoparticles through an Ostwald rippening process.3e,4a As the reduced Pd atoms are largely confined to the vicinity of the outer surface of the sacrificial templates, the morphology of the Co nanoparticles determines the corresponding hollow Pd nanostructures. Therefore, the fabrication of Co nanoparticles is a critical aspect in the preparation procedure. Another interesting feature of Co nanoparticles is their magnetic property, which has been studied in depth. As indicated in the literature, the Co nanoparticles in the present study are ferromagnetic at room temperature9 and magnetic interactions between them play a crucial role. In addition, the interaction between the citric acid capping agent and the Co nanoparticles is important. This interaction relates to the adsorption of citric ions on the Co nanoparticle surface, thereby inhibiting the growth and aggregation of Co nanoparticles through electrical double layer repulsions between the negatively charged cobalt nanoparticles. The result is an equilibrium between the magnetic attractive force of the ferromagnetic Co nanoparticles and repulsive steric force of the adsorbed citrate ions. In the case when the concentration of citrate ions is insufficient, this equilibrium will be broken, the dipole-dipole interaction is stronger than the repulsive steric interactions caused by citric acid, and the result is particle aggregation. It is reported that single domain behavior is possible when Co nanoparticles are considerably smaller than the theoretical critical single domain diameter (ca. 55 nm),14 i.e.,
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Figure 3. Various TEM (a-c) and SEM (d, e) images of nanotube-like Pd nanostructures at differing magnifications.
all of the atomic magnetic spins of the nanoparticles are coupled in the same direction and the nanoparticles behave as a single magnetic dipole. In this case, the Co nanoparticles will form chain-like 1-dimension (1D) nanomaterials to minimize the magnetostatic energy. The magnetic dipoles should be in an alternating up and down arrangement in this chain-like assembly.15 Nanotube-like 1D Pd nanomaterials can be fabricated by the reaction between chain-like Co nanoparticles and PdCl42-. These nanotube-like 1D Pd nanomaterials are detailed in Figure 3a-c. These show various TEM images of different magnifications of the as-synthesized nanostructures. It is evident that the center portions of these 1D nanomaterials are lighter than their wall edge, providing evidence of their hollow structure. The high-magnification TEM images in Figure 3b,c reveal the detailed structure. These 1D hollow nanotube-like configurations (ca. 60 nm in diameter) are formed by the connection of hollow nanoparticles and are robust enough to survive both centrifugation and ultrasonic treatment. It can be seen that the diameter of the nanotube-like configurations (ca. 60 nm) is greater than the theoretical single domain for Co (ca. 55 nm). This can be tentatively attributed to two possible phenomena. First, in the reaction process the Co atoms may move outward such that their reaction with PdCl42- forces the Pd particles to grow outward, making the central diameter size larger. Second, as indicated by the TEM and SEM images, the surface of raspberry-like particles is rough and composed of nanoparticles, the presence of which can make the size (60 nm) bigger than the theoretical single domain (55 nm). The low-magnification SEM image presented in Figure 3d
indicates that large-scale nanotube-like 1D nanomaterials several microns in length have been fabricated. Figure 3e is the highmagnification SEM image of such a material. It is evident that the surface consists of smaller nanoparticles, similar to that of hollow raspberry-like Pd nanoparticles. Figure 4a displays the HRTEM image taken from the surface of an individual nanotube-like 1D nanomaterial and shows clear evidence of a polycrystalline structure. The arrows show the lattice spacing is ca. 0.223 nm, similar to the Pd {111} lattice spacing. In addition, Figure 4b provides the energy-dispersed X-ray analysis taken from a random assembly of nanotube-like 1D materials, again indicating the Pd nature. The facile synthesis of these hollow raspberry-like Pd nanoparticles means they are attractive for a variety of applications, in particular the Pd/hydrogen interaction. The response of these hollow raspberry-like Pd nanoparticles toward protons was next explored and compared with those of solid Pd nanoparticles (diameter 3 nm), Pd microparticles (diameter 1-1.5 µm, Aldrich), and a planar Pd macroelectrode (diameter 3 mm). The TEM image of solid Pd nanoparticles and SEM images of Pd microparticles are shown in Figures S1 and S2, respectively, of the Supporting Information. The various Pd particles were cast onto the surface of a boron-doped diamond electrode and their electrochemical responses examined in 1 M H2SO4. Figure 5 shows the N2 adsorption/desorption isotherm and the pore-size distribution (inset) of hollow raspberry-like Pd nanoparticles. According to BET (Brunauer-Emmett-Teller) analysis, the total specific surface area is 23.6 m2 g-1. The BJH
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Figure 4. (a) High-resolution TEM (HRTEM) images taken from the surface of an individual Pd 1D hollow nanomaterials; the arrows show the lattice spacing of 0.223 nm. (b) Energy dispersed X-ray analysis taken from a random assembly of such materials.
Figure 5. Nitrogen adsorption/desorption isotherm of hollow raspberry-like Pd nanoparticles. The pore size distribution plot (inset) was calculated by the BJH formula in desorption branch isotherm.
(Barrett-Joyner-Halenda) pore size distribution shown in the inset of Figure 5 indicates that the Pd particles have various nanopores in the range of 3-60 nm. This can be assigned to the mesoporous structure of the shell, the interparticle spaces of these raspberry-like particles, and the hollow cavity of these particles. The low surface area can be attributed to aggregation of these particles, in agreement with the SEM images detailed in Figure 1c,d. This is due to the citrate capping agent used in the present study. However, the size distribution result clearly demonstrates the nanoporous structure of these unique particles. Figure 6 details the 1st and 10th cyclic voltammograms of four different Pd electrode layers at the scan rate of 100 mV s-1 when placed in 1 M H2SO4. In these cases, the scans were
started at +0.4 V, swept to -0.4 V, and cycled back to +0.4 V. Figure 6a was obtained using the hollow raspberry-like Pd nanoparticles, Figure 6b using solid Pd nanoparticles, and Figure 6c was derived from Pd microparticles, while Figure 6d was recorded at a planar Pd macroelectrode. Note the different current scales in each figure. Parts a and b of Figure 6 have the same catalyst loading of 1.5 µg, while the catalyst loading in Figure 6c is 15 µg in order to obtain a significant response. It is evident from these cyclic voltammograms that, under identical conditions in 1 M H2SO4, the hollow raspberry-like Pd nanoparticles exhibit a significantly higher and more stable response than that of the solid nanoparticles and the microparticles. The response in Figure 6a shows the presence of a small reduction
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Figure 6. The 1st (dashed line) and 10th (full line) cyclic voltammetric responses of (a) hollow raspberry-like Pd nanoparticles, (b) Pd nanoparticles with average diameter of 3 nm, and (c) Pd microparticles (diameter 1-1.5 µm, Aldrich) all deposited onto a 3 mm boron-doped diamond electrode and (d) a 3 mm Pd macroelectrode when placed in 1 M H2SO4 (scan rate 0.1 V s-1, scan range from +0.4 to -0.4 to +0.4 V). The catalyst loading of a and b are 1.5 µg, whereas the catalyst loading of the microparticles is 15 µg.
wave at ca. -0.15 V and major reduction waves at ca. -0.35 V and two oxidation waves at -0.20 and -0.04 V, consistent with previous results.8 In contrast, the responses of the solid Pd nanoparticles, the microparticles, and the Pd macroelectrode, presented in Figure 6b-d, showed no well-defined reductive peaks and a single broad oxidation wave at +0.12, +0.10, and +0.30 V, respectively. Furthermore, the responses obtained from these three forms of Pd show an unstable response with repetitive scanning with an enhancement in the currents recorded as the potential was cycled. The results presented in Figure 6 clearly demonstrate the superior response of the hollow raspberrylike Pd nanoparticles compared to the other forms of Pd studied. These results can be rationalized in the following manner: it has been demonstrated previously that electrodeposited Pd nanoparticles show two distinct oxidation waves corresponding to the oxidation of both Had and Hab as the numbers of surface and bulk Pd sites (into which the hydrogen can diffuse) are comparable in these small particles.7,8 It can be envisaged that this is the case with the hollow raspberry-like Pd nanoparticles, where the oxidation wave at -0.20 V corresponds to oxidation of Had and the wave at -0.04 V corresponds to the Hab oxidation. Similar behavior might be expected for the solid Pd nanoparticles, where the numbers of surface and bulk sites are also comparable.7 However, this is not the case for the solid Pd nanoparticles, with the oxidation peak being found at potentials not dissimilar to that of absorbed hydrogen. This unexpected behavior is tentatively attributed to the PVP capping agent influencing the Pd adsorption/absorption properties of hydrogen atom. The strong affinity of the PVP capping agent with the Pd nanoparticles hinders the diffusion of protons to the Pd
surface and therefore inhibits both electron transfer from and diffusion of hydrogen atoms into the Pd. Furthermore, in contrast to the hollow raspberry-like Pd nanoparticles, it was found that the Pd nanoparticles were unstable over time. The experiment detailed in Figure 6a,b was repeated with a solution prepared 24 h earlier, and enhanced oxidative peak currents were observed with the solid Pd nanoparticles; in contrast, the hollow raspberrylike Pd nanoparticles showed little variation in peak current, demonstrating the stability of these particles. These results demonstrate the significant advantage of using the hollow raspberry-like Pd nanoparticles compared to the solid Pd nanoparticles for proton/hydrogen detection. First, the hollow nanoparticles have enhanced stability, and second, a weak capping agent, citrate ions, inherent in the fabrication process of the Co nanoparticles, was used: strong capping agents usually contribute to the formation of small nanoparticles as they tightly cap the nanoparticles, but with a lower active surface area. However, unlike the strong capping agent used in the solid Pd nanoparticles synthesis, in our protocol, the weak capping agent allows the growth of larger Co nanoparticles, and the corresponding newly fabricated hollow raspberry-like Pd nanoparticles thereby consist of smaller nanoparticles, which are weakly capped. Therefore, the surface area available for hydrogen adsorption/absorption is greater for the hollow raspberry-like Pd nanoparticles than for the solid Pd nanoparticles. In the case of the Pd microparticles and macroelectrode, the ratio of bulk to surface sites is raised considerably; therefore, the voltammetric responses are entirely dominated by the oxidation of absorbed hydrogen.5 Furthermore, the enhancement in the response with repetitive scanning for these two electrodes
344 J. Phys. Chem. C, Vol. 112, No. 2, 2008 might be tentatively attributed to the accumulation of hydrogen atoms within the Pd lattice. In contrast, the highly stable response of the hollow raspberry-like Pd nanoparticles might be due to the rapid filling of the Pd lattice by hydrogen atom in these hollow nanoparticles. It is worth noting that the electrochemical response of hydrogen is closely related to the size and structure of the Pd particles, as shown from the cyclic voltammograms in Figure 6. A more detailed understanding of the relationship between the electrochemical response of hydrogen and Pd nanostructures is currently under investigation. However, a remarkably enhanced, more specific and stable electrochemical response of hydrogen is achieved using hollow raspberry-like Pd nanoparticles, which can be directly related to their unique structure. Finally the effect of scan rate (0.025-0.5 V s-1) on the voltammetric response of the hollow raspberry-like Pd nanoparticles when placed in 1 M H2SO4 was studied and is shown in Figure S3 of the Supporting Information. It was found that, by increasing the scan rate, an enhancement in the oxidative current attributed to the absorbed hydrogen oxidation process was observed; conversely, by decreasing the scan rate, the adsorbed hydrogen oxidation process dominated. This can be tentatively attributed to hydrogen diffusion process within the Pd lattice. At slower scan rates, the hydrogen atom diffuses into the bulk and is allowed time to fill the Pd lattice such that the bulk sites and surface sites become saturated with hydrogen; conversely, at faster scan rates the hydrogen species can only diffuse into the bulk and there is no time for the Pd bulk and surface sites to saturate such that only oxidation of adsorbed hydrogen is observed. Conclusion In summary, we have demonstrated a facile procedure for the controllable synthesis of hollow hierarchical Pd nanostructures with Co nanoparticles as sacrificial templates. It is the contribution of small nanoparticles on the surface and hollow structure that endows these weakly capped particles with high electrocatalytic activity. These unique properties allow the hollow raspberry-like Pd nanostructure modified electrodes to exhibit stronger, more specific, and stable response in the electrochemical detection of proton/hydrogen compared to Pd nanoparticles, microparticles, and macroelectrode. In the light of the wide applications of Pd, these hollow hierarchical Pd nanostructures combine the advantages of easy separation of microparticles with the high activities of nanoparticles. This will promote applications in various fields, such as hydrogen storage, electrocatalysts, sensors, and heterogeneous catalysts in carboncarbon coupling and hydrogenation reactions. Supporting Information Available: Additional TEM and SEM images and cyclic voltammetric results. This material is available free of charge via the Internet at http://pubs.acs.org.
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